I bought an adapter plug for connecting it to shore power that Supertramp recommended, but I can't recall where I put it. It's warm enough now that it's not needed, but it's something I'll look at going into next winter period.
For those not familiar with the optional "plus" version of the Truma Combi, it contains an electric heating element that can be powered by 120v shore power, but it receives its own exterior wall plug separate from the camper's battery charger plug. That Combi Plus plug, on the camper exterior, is a NEMA 5-20P (P stands for plug) rather than a NEMA 5-15P like the battery charger plug. NEMA 5-20 plugs have one prong turned sideways, rather than both prongs being parallel, and indicates it should be plugged into a 20amp circuit rather than 15 amp circuit. You can use an adapter with a NEMA 5-20R (R is for receptacle; aka outlet) and ensure you only plug into a 20amp 120v outlet if you plan to use the full potential (highest setting) of the electric heating element. This is because the combi plus can draw up to 1,700 watts (14.2amps on 120v) and best practice is to limit continuous draw to 80% of a circuits rating, which would be a max continuous of 16 amps on a 20amp circuit. On a 15amp circuit, max continuious draw should be limited to 12amps (1,440 watts).
There is the option to run the Combi plus on 'Operating Level 1' which draws a max of 850 watts (7.1amps on 120v). It is perfectly fine to draw a continuous 7.1 amps from a 15amp circuit since it is well below the 80% rating which is 12amps.
Whether you plug the combi into a 20amp circuit or, use and adapter to plug into a 15amp circuit, the caveat on any circuit is there could be other loads on the same circuit. For example, if your camper is plugged into an outlet in your garage and there is another outlet on the same circuit with a garage fridge plugged into it, or someone was to plug in a power tool or heater to the same circuit, the circuit may experience sustained draw over the 80% circuit rating level or simply be overloaded and trip the breaker or blow a fuse in the service panel.
...I did get the multimeter out, but it made me even more confused to be honest. All 4 batteries reported 2.94V. I measured from the positive bus bar to the shunt, and that also was at 2.94V. The BatteryProtect measured 11.31V. But the Victron App shows the BatteryProtect battery voltage at 13.49.
From your earlier post, it makes sense that the SmartBatteries, SmartBMS and outbound (load) Battery Protect are not showing up if your battery bank is dead. The inbound (charger) Battery Protect is likely showing because it is getting powered because your charger is plugged in. If you unplug all charge sources, it's likely to eventually disappear from your Victron app menu as well. That seems like a big hint.
I don't think charge is getting to your batteries at all.
You are testing voltage with the charger turned on, correct? That is a good way to confirm where the charger voltage is passing and where it is not. You mention testing at the Battery Protect. There are two of those, but because you had voltage, it sounds like you tested on the charger side if the Inbound Battery Protect. The other Battery Protect will be the Outbound Battery Protect (Load going out of the battery). One battery protect turns off to prevent outbound current from the batteries and the other turns off to halt inbound current to the battery (from solar, shore power charger and DC/DC charger). In a situation where the charger is on an Inbound Battery Connect is blocking current to the battery, you'd have bulk or float charger voltage (12v+) on the main positive busbar (assuming the charger is connected directly to the busbar, charger is on, and any breaker or fuse on the outbound charger conductors are flowing). You'd also have charger voltage right up to the charger side of the Inbound Battery Protect, but if it was triggered to shut off, voltage/current would not be present on the battery side. With dead battery bank, your BMS isn't able to tell the Battery Protects what to do so I imagine they err on the side of caution and shut off current unless the smartBMS instructs them otherwise.
I think you need to apply 12v power directly to the battery terminals to wake up the batteries and external smartBMS so it can correctly give instructions to the battery protects. To do so, you'll need a battery charger that has a wake-up function (most victrons, but also many other LiFePO4 battery chargers), or a 12v power supply, or any 12v battery (use smaller gauge wire to limit current in-rush. Don't use Auto Jumper Cables, for example. Just some 16 to 12 gauge wire will limit the in-rush. But you may still get a spark so eye protection, etc.)
Connect any of those sources directly to the Victron smartLithium battery terminals which will energize the smallBMS. It won't take much current but might take a little bit of time for everything to power up and start talking. Once they wake up, the smartLithium, smartBMS and both battery protects should be available in your Victron app menu. Just leave the shore power charger plugged in as it will immediately start charging once the batteries wake up.
Another thing to check, if you have not already, is that all communication cables are firmly seated at all connectors; Connectors between all your batteries and connectors between batteries and smallBMS) and then also check the cables running from the smallBMS to both Battery Protects). Lack of communication may not allow the inbound Battery Protect to allow current to flow to battery from any of your charge sources. But I think your connections are likely fine and your batteries just need to get a wake up with a 12v source directly to the terminals. I should have originally recommended you do that as it's the most likely issue, but didn't think to suggest it until I heard more and thought through it.
