I would most definitely check your on-borad trailer battery... If it is requiring alot of charge for one reason or another if may be wanting to see alot of DC current... Check the fluid levels and the VDC level... If fully charger it will show 12.6-7 VDC... If it has a shorted cell and only reading 5-6VDC across the terminals then when you hook up it may want to draw alot of current from the truck side enough to blow a fuse...
Does your ceiling lights woprk ok when you are just using your battery
My 2008 popup trailer came with those type of converter/charger unit and that was one of the first things I replaced with a modern converter/charger unit with many circuit breakers and ATC fuses positions... Now everything in my 12VDC Distribution has its own fuse....
Its going to be hard to figure what is doing the short for you when four or five 12VDC circuits are beiing under one larg fuse...
All of my three or four 12VDC zones behind the converter box was twisted together under one of those large YELLOW twist lock connectors... If you can track down which group of iwisted together cables that gets fed by that 20AMPS fuse you can un-twist the yellow connecot and hopefully figure out which is draining the big current...
Will take some digging around for sure...
A clamp around DC CURRENT meter would be a great tool to use for this searching...
Roy Ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS