You could do a simple test to see how much voltage is getting to the rv battery. Disconnect house 12v power. First discharge the RV battery to 50% charge so the alternator could throw out more current. Then
Start tow vehicle.
Measure voltage at tow vehicle battery.
Measure voltage at RV battery.
The difference is loss.
You will have to drive a long time to recharge the battery. It will charge up to 90 some% pretty fast and then take a couple hours to top off.
My trailer would sit and the battery would be low at the outset of a trip. Then we would only drive an hour to the destination arriving with a 95% battery. We addressed the natural discharge with a 4 stage charger at home, and now have solar
charging for in camp needs. Life is good!
I used 10 ga. for the vehicle charge line, but never measured the loss. It isn't important now that the solar
is in the picture.
PS I checked out your website. Beautiful work!