I don't know how your trailer is wired, but on my 2001 trailer the disconnect switch disconnects both the batteries and the converter from the 12V
fuse panel. As a part of the 12V
distribution system there is a "resettable fuse" or circuit breaker that is thermally activated in the line to the batteries. If that was bad, everything else would work, but not the trailer batteries. You said the battery disconnect fuse was good, but said it was a 5 amp. Did you mean 50? If so, then ignore the above.
"You cannot reason someone out of a position they have not been reasoned into"
Al, K5TAN and Missy, N4RGO
2002 Classic 30 Slideout
2001 Safari 25 RB Twin (Gone, but not forgotten)
2013 Dodge 2500 Laramie 4x4 Megacab Cummins
WBCCI 1322, TAC FL-39, AIR 82265