A simple cure is Heavy Duty Jumper Cables 12 feet long.
we use 2 group 27 batteries although the 6 volt GC2 is a better plan .
I have installed AGM (absorbed glass mat ) group 78 batteries in my duramax .
We run from all 4 batteries while boondocking .
since 1995 we have done a 9 day campout with our camping circle of friends off the grid .
we park the trailer , turn the pickup around and run from 4 batteries. 2 rv batteries and two batteries in our gmc diesel .
the pickup gets driven every 2nd or 3rd day .
when we return we hook up all 4 batteries with the jumpers .
we do not run the engine to try to bring the RV batteries up .
the 4 batteries equalize via the HD jumpers .
we do charge up our batteries on 120V at home before and after each trip , while packing / unpacking .
we do not own solar or a genny .
nor do our friends .
Exide EDGE AGM batteries group 78 cost me $ 140 each at the local BiMart .