Here is a typical 30AMP electric distribution chart that might help...
I'd go for loose terminal connection on your battery terminal...

My 2008 OFF-ROAD POPUP came with the simple ELIXIR converter/power distribution unit that only had one 120VAC circuit breaker and four 12VDC fuses. All of the circuits feeding this box was divided up into many groups using those large YELLOW twist on wire nuts. What a night mare those were. I eventually changed all of that out for a more modern WFCO 8900 series Power distribution center panel. Now I have up to nine 120VAC circuit breakers (One full size and eight half size) and eleven 12VDC fused positions. Much better layout...

After fighting with the new WF8945 converter/charger unit I changed that out for a PD9260C converter/charger unit. Having good luck the past five years running this setup.
Roy Ken