Using 12V while hooked up to the tow vehicle

sofraj wrote:
When I went to install the battery to winterize, nothing 12V would work. Long story short, 12V in the camper will only work when the tail/marker light cable in attached to the tow vehicle (2012 Toyota Highlander).

Your trailer is wired with two separate 12 vdc lighting circuits and neither has anything at all to do with the other - 1) the exterior lighting (running lights, brake lights) and 2) an interior system which includes interior lighting. Anytime there is power on Pin 4 of the trailer's Bargman connector, be it supplied by the trailer battery OR the trailer's converter when plugged into shore power OR the tow vehicle that interior 12 vdc system will be operational. When you say "12V in the camper will only work when the tail/marker light cable in attached to the tow vehicle" what is actually happening is that by connecting that tow vehicle cable to the trailer you're supplying + 12 vdc to Pin 4 of the Bargman connector and thereby supplying power to the trailer's interior 12 vdc system, including the lights. Since this seems to be the only 1 of 3 potential sources of power for this circuit that works that would suggest you probably hooked up the battery incorrectly, thereby blowing the reverse polarity fuse(s) in the converter ... so check for correct battery connection and check the fuse(s). You may have also tripped the circuit breaker connected inline with the cable connected to the battery's positive terminal ... if it's an auto resetting CB it will reset on it's own once the fault is removed, if it's a manual reset you'd have to reset it yourself. If the manufacturer installed a fuse instead of a circuit breaker then you'd obviously have to replace the fuse, of course once the cause of the fault is removed.