Haven't recently read whole thread so maybe missed something and don't know what your experience there is, but on the dual battery thing: for RV's and some truck setups we use a Battery Isolator. These split the charging so that the main vehicle battery is always fully recharged first, then when it's fully charged the charge output from alternator automatically switches to the auxiliary battery until it's fully charged as well.
This way your primary battery for starting is always ready to work and the charging of auxiliary doesn't rob charging from main. Without these Isolators, in an RV, you can accidently discharge auxiliary, then maybe have some hard starting for the main and next you know everything is dead. So it's good backup protection. Isolators are a little pricey but if you shop around a dual system isn't that bad.
Anyways, nice build!