Don't over think it, just get 2 new deep cycle batteries and rock on. If you don't use the motorhome regularly, hook a battery charger up every now and then to maintain them. You have a separate battery to run the engine electrical, but when the RV is going down the road it will charge all 3 batteries. The coach batteries only burn the DC lights inside, run the fridge when it switches to propane while going down the road, runs the water pump for the sinks and toilet, and fires the water heater when you cut it on. It only does all that when you aren't plugged into shore power.