One more important consideration. I note you are planning on installing an inverter.
Depending on the amperage/wattage of the inverter, you will be needing some very heavy cables from the battery bank to the inverter. The inverter you choose will likely suggest the gauge cable needed to feed it. Please consider that the extra battery in your wheel well will be some feet away from your in-camper house battery pair. That means when you install your parallel cables to connect all your batteries in to one parallel system, you will have to consider they will need to be at least as large as the cables needed to feed your inverter. They certainly can’t be smaller. Don’t forget if you have a shunt for a battery monitor the negative for the inverter has to come off the load/downstream side of the shunt. All in all, between the battery bank, the extra battery, the shunt and the location of the inverter, you are likely to have surprisingly long runs of cable which means you could have significant voltage drop. There are online DC voltage drop calculators that you can use to check that. You will have to use the relatively high amperage of the inverter when you enter your data into a voltage drop calculator and that will really increase the voltage drop. And remember that the charging voltages from your solar charger and your Onan/converter are measured in tenths of volts so it is easy to have too much voltage drop with too small cables that affects the charging voltage ultimately seen by the most distant battery. Having that extra battery at some distance from your house battery means that it may see totally different voltages then the house batteries if your cables are not big enough.
Long story short, you only install the cables once. Whatever the inverter suggests for a cable gauge to feed it, you may have to upgrade one or even two sizes heavier (larger cable = smaller gauge number) to make sure you totally minimize any voltage drop in your system. Figure your voltage drop length from the inverter, through the shunt and through all the parallel cables to the furthest battery. And I would size the cable so the voltage drop between your in-camper batteries and your wheel well extra battery is negligible. I would make make all the cables in the parallel system and to the inverter all the same size – defaulting to the size needed for the longest run.
And I would suggest ordering marine cable for that purpose – it is much finer stranded and is so much easier to route and install IMO. You can order online any size cable and color from Greg’s Marine Wire Supply. They sell it by the foot and also sell lugs by the piece, heat shrink by the foot, and solder pellets by the piece. I use solder pellets for the heavier cable lugs on my rig so I did not have to find/borrow or buy a hydraulic crimper. Stripping the marine cable does take some care to avoid cutting the fine strands but I do it with a utility knife, take my time, and rarely cut a strand.
Just something to consider when adding an extra battery and hooking up to an inverter.