If you are having problems with 12V curcuits, look to the converter fuses. if the problem is charing while towing look to the circuit breaker up near the battery.
Check the connectors, some rocket scientist thought it would be OK to use an oversized ring connector on a terminal that is is half the size of the and that jamming the edge of the over sized terminal under the nut.
Another problem is where the circuit breaker installed in the perfect spot for water penetration. I went down to the local electronics hobby store and bought a project box. Now my breakers are nice and dry out of the elements and protected from road debris.