Using an ohmeter may just drive you crazy. You have a built in signal tracing component. Your truck batter.
Start by checking the outlet on the truck with a test light versus a voltmeter. A voltmeter can fool you however your analog meter is less likely to.
Check that all functions are good. If you find the backup light function missing then find the fuse that popped. It is easy to turn one of those plugs over just enough to pop a fuse.
With that sorted out you can move on to the battery that is probably sick. If you have a severe short things will melt and fuses will blow. I doubt you have a severe short. You can lift the battery lead off the camper, with the things not plugged into shoreline or the truck, and put your test light in series with the cable. If it lights, you have a decent draw and then need to chase that.
Simply pull each fuse in the panel to isolate the circuit that is shorted if your find the test light is glowing.
