Don't be so sure an off-the-shelf battery pack isn't basically the same.its a big ugly plastic box stuffed full of circuit boards and hot melt glue.
And carry a LI jumpstarter kept fully charged in the glovebox
I have the Suaoki G500. I also have a clone of the Suaoki 400. I have used the 400 on several week long trips and it easily kept my refrigerator charged overnight. I was moving often so I simply kept the 400 plugged in to my starter battery via cigarette lighter plug. I also have 200W of solar panels. When there was sun, I connected one panel to the 400. I keep one movable and flexible 100W panel in reserve to use in certain locations to give it more optimum angle to the sun. I can simply establish the place for the panel and then move the 12 lb. battery close to it.
In general, I'm happy with the arrangement but I wanted more reserve power thus I purchased the G500. I haven't tested this setup yet but I feel that it will provide all the power needed under most conditions. I'd like to keep the movable panel but add another permanently mounted 100W unit. One battery will stay in the cab and the other will always be connected to the 200W on the roof. I keep the refrig. in the cab where I either charge it off the truck or use the portable panel. I've never had a problem with the 400 timing out. I still don't have enough experience with the G500 to determine if it times out.
This whole setup eliminates a lot of complications and I feel it suits my minimalist style. I like not having to setup an electrical panel, solar controllers, USB, etc. I will charge and/or power all my camper lights directly off the G500 and thus eliminate a lot of wiring. The only time I'll be drawing from my truck starter battery is when the engine is running. I can easily move the lithium batteries around and substitute one for the other if needed. I'm OK with the lower cycling life(compared to LiPo) because the batteries will still charge after the 500 or whatever time life and after 500 cycles if the lower capacity isn't enough I'll simply buy one more. I've dealt with heavy golf cart batteries with the heavy cables and all the complications before on boats. I decided to go simple and light with my truck. It's nice to have your electrical setup isolated from the truck system.
Depending on the alternator, I think it may need something to limit the current, but that's it.If I were making a lightweight fridge powering system, with no solar, here would be my approach.
Buy or build a 100AH LFP pack. DIY would be around 500$, off the shelf 700-900$. There are a couple discussions in this forum, and some youtube how-tos. This will have a basic BMS (disconnect and cell balancing). From there get an 100A rated relay attached to the vehicles charging system (at a high current point). Add a fuse block, and volt meter for monitoring.
At this point you have a battery system which does not suffer from partial charge damage like lead. It will easily accept 30-50A of charge. This will allow your primary charge source to be the alternator. Add a disconnect switch for storage.
Correct. Just for running the controller, fan, & spark. I've run it for 6+ hours from a computer UPS with less capacity, so I think this will be great for the purpose.It won't heat much water, but if you mean the spark & pump, for a propane instant HWS sure.