Dan,
some people make things that are simple hard and it doesn't need to be that way.
I have been doing what you are wanting to do for many years and it works great. I have a 06 Tundra with trailer package, I installed a 7 pin RV socket in the left rear of my truck and have a pig tail with a connection to the extra battery.It will charge the battery up if you are driving a few hours not just short trips. My pig tail is only 2' long and 10 Gage wire. We did a 3 week trip this last spring and only camped with elect one night, we have a 17' Casita and we used lights, water pump, heater and the fanatic fan as needed and used our shower too.
