My best guess is that the electric heating element has a short to ground. It is relatively easy to isolate the heating element. 1. Make sure that your 110 volt power to your camper is off. 2.Disconnect the wires at the heating element. 3. Make sure the wire ends do not touch anything--electrical tape if you want to be cautious. 4. Now test to see if switching on the electric to the water heater blows your blue fuse (with 110v to the camper still off). If it does, the problem is in you wiring, and not the heating element. 5. If it does not blow the fuse, turn on the 110v power to your camper and try turning on the electric to the water heater again. If the fuse does not blow, then replace the heating element.
My guess is that you have a short in the element. With the wires to the element disconnected, you should not have any resistance between either contact where the wires attach and the body of the water heater. Heating element replacement is one of the easiest and cheapest things to do.
If element ends up being good, next best guess is that the problem is either in the 110v switch or the wires between the switch and the water heater.