Update on the battery decision. When I removed and stored my CAMPX for the winter, I pulled out the 100AH AGM with the intent of replacing it with lithium by spring. It was a VERY tight fit removing it. I saw they have changed the layout within that compartment and moved the shunt and breakers over to the partition wall to which the REDARC is mounted. That seems like a great move as it will allow for more room removing and replacing a battery from the battery box. Love the constant refinements.
After measuring everything, I determined I could only fit one 100AH lithium battery rather than two in the water tank cabinet even if I removed the battery box. I'd wanted two batteries which would allowed me to drive a larger inverter since max continuous discharge current would be split over two batteries. Most 100AH lithium have a 100A 'max continuous discharge current' rating so a 100AH can run an inverter of about 1200w; but two 100AH with 100A draw could run a 2400 - 3000w inverter. Ah well, 1200w inverter will suffice. I'd planned on Battleborn but was open to other brands as long as the quality was there. Several reviewers had recently tested and torn down the SOK brand battery and found construction quality to be high so I decided to try one of this brand. Bonus is that they are significantly more economical than Battleborn (though they are imported rather than made in the USA like Battleborn). Besides the price, the main deciding factor was they offer a 200AH 12V battery. Same footprint as the 100AH but just taller. But, since they are a single battery, the max continuous discharge current is the same 100A as the 100AH battery so still limited to a 1200w inverter unlike going with two 100AH batteries.
SOK Lithium:
100AH specs: 11.42 x 7.87 x 7.87", 26 lbs.
200AH specs: 11.42" x 7.87" x 11.02", 47.6 lbs.
I looked at the two sizes/types of Battleborn 100AH batteries and there is no way to two of them in my battery box. Battleborn does not make a 200AH, so I went with a SOK. Next batch was 30 days out when I ordered and mine is now ready and will be shipping. This will give me around 180 - 190AH usable vs the 50AH usable from my stock 100AH AGM so this will be a huge improvement (nearly 4x). We use 20 - 30amps per 24 hours so if I utilize 180AH of the new battery, we'll be good for 6 to 9 days with no input. This is with our current typical use. I do intend to add an inverter and will use our induction cooktop and start charging our eMTB and electric offroad mini-motorcycle off if it so with additional capacity will come increased use!
I also found a deal on Renogy 175watt flexible solar panels over the holidays and bought two. I'll sell our Renogy 100watt portable suitcase. It is heavy and a bit awkward to set up and travel with so figure I might as well find another solution to make it more pleasant. With 200AH battery, I won't bring solar panel(s) with very often, but have other projects where I'll use the solar panels as well (a Bluetti AC200 Solar Generator), so I will not permanently mount them on the CAMPX roof. That means I will not need to add roof-top solar wiring (another bonus). I can throw one panel in the cabover bed or pickup's back seat if I want to bring it with, which will be rare unless we expect to do a lot of eMTB or eDirtbike charging. When I do, we'll lay it on the roof, the ground, across the windshield, etc. as appropriate and plug into the existing side solar plug.
Won't bring solar with very often because we typically do 2 or 3 day trips and 200AH will be plenty with no input the entire trip. On all our longer trips, we drive every day or two. I don't like to stress my alternator with fast charge rates, but on a 4 our drive (typical when road tripping from spot to spot), I am comfortable recharging as we drive if I set the REDARD to between a slow 4amps (minimum REDARC setting) to 10 amps per hour as to not stress anything. If an emergency fast charge is needed, one can set the REDARC all the way up to 30amps, but I don't foresee, and prefer to not be, doing that from the truck's DC. Nice to have the 30amp charge rate if one needs a quick charge off shore power, though. I also have a manual switch, under the hood, to disconnect the pickup DC from the camper. I did that in anticipation of going lithium someday as lithium batteries do not like to be stored at 100% especially in heat and I wanted a way for the truck to NOT be charging the battery. My intent is to never charge off the pickup's DC unless absolutely needed. When driving home, I don't want the truck to recharge the battery. When we arrive home, I'll let the battery sit at whatever state of charge we got down to on the trip. Ideal lithium storage SOC is between 40% and 60%. Will wait to charge it back up to 100% until the day before leaving on a trip (be that with AC shore power or by parking outside and hooking up the solar panels, which are my preferred methods of charging the battery rather than DC from the truck alternator).
Still deciding on what inverter and where to place it. Leaning towards the Victron Phoenix 12V/1200. I think I'll be able to mount it on top of the battery box which would allow the shortest 12v run. I'll leave the under-sink 120v outlet tied to shore power but will change the corner counter-top 120v outlet to be powered by the inverter. Alternative inverter locations are in the under-fridge cabinet or in the under-sink cabinet, but like mentioned, would much prefer to mount in the underseat water tank compartment somewhere to keep DC cable run short.
[Update on 2/5/2021: The SOK 206AH battery arrived, and while it will fit in the battery box itself, there is no way to get into the battery box due to the placement of the battery box within the under-seat cabinet, water tank location, and shunt/breaker locations. While I had premeasured and knew I'd have to cut out a section of the aluminum battery box to install this particular battery, I started a cut and quickly realized it would be a long and messy grinding/cutting process so lost all enthusiasm to make this work by cutting up the battery box. I even considered cutting an access hole in the front exterior wall and installing an external door (or patch) to allow a battery to be inserted into the battery box from the outside, but decided against it so never contacted Overland Explorer to see if there would be any structural concerns with that. Next bit of research was to see if the smaller 100AH SOK battery would fit with less or no modifications. It has the same footprint as the 206AH but is a few inches shorter. I used cardboard to model a lifesized blank. No go. It is the 7.9" width that makes the SOK batteries impossible to fit. Battleborn are 6.88" wide so the same width as the frustratingly tight-to-remove-but-still-possible stock AGM. So I modeled another blank based on the Battleborn 100AH and it will juuuuuuust fit. So I ordered a Battleborn and am awaiting it to arrive. Yes I could have repurposed the under-fridge cabinet for one or more batteries, and would have been fairly easy to wire, but we would lose a valuable storage area, so wasn't willing to consider that. We'll settle for a 100AH battery instead of the 206AH that we were so excited to have.]