I'm embarrassed to even say this, but I went camping one weekend, and every time I connected the battery the fuse would blow at the battery or at the fuse box.. after I ran out of fuses I just ignored it until I got it home.
Turns out I had the battery connected backwards.
Assuming that's not your problem, you'll want a multimeter. It sounds like there's a short in the plug between your tow vehicle and the camper. The plugs vary, but you can use the multimeter to test for continuity between the pins. Consult a wiring diagram for your plug type to determine which pins should be continuous.
1995 Toyota 4Runner SR5 4WD 5 spd
2002 Eagle Summit 12