measuring resistance in a wire with a short
You have narrowed down the short to one circuit. I'll assume the camper trailer  is conventional automotive negative ground with +12 fused. 

So there is a short between the circuit you have identified and ground. 







simulate this circuit &ndash; Schematic created using CircuitLab

When the fuse you have identified (F5 in my schematic) is removed  you should have no voltage relative  to ground on one side of the fuse holder (and 12V on the other). 

At this point you have identified the wire that is shorted and there is not much else you can do with normal test equipment. The exact resistance and the wire gauge will give you sort of a hint as to the location, but not much more than a hint. 

What you have to do is to trace the wire from that fuse holder to wherever it goes and fix the short (maybe it is pinched where it goes through a door hinge or the insulation has cracked where there is pressure on it). You should be able to figure out what it powers since that is the thing (or  things) that will not operate with the fuse removed- everything else should work. Maybe there are markings that are a hint on the fuse box. If it is just one thing you could consider cutting the wire at the far end and bypassing it with a new wire. Of course  if you have a proper manual for the wiring it should show you the wire path, color codes, splice locations, ground connections and so on for the entire harness. 

If you can provide some photos and detailed information the mechanic SE could be of help- I'm sure this problem comes up in maintenance of classic  cars.