Unless FR changed the set up that 10 gallon water heater is hard-wired. One of two possibilities. The wire has come apart at the water heater, I know first hand that the wire nuts can come loose from travel. In my case it was in a junction box for the receptacles. Wires weren't twisted together and eventually, with the load the wire nut became a little toaster and melted away the the plastic, causing the wires to come apart. The other possibilty is it came loose in the electric panel. You could check for continuity through the wire by removing the breaker for the water heater and removing the neutral at the panel, turn the outside water heater switch to on and then check between the black and white wire. Of course you want to do this with the coach unlugged. If everything is good you should have continuity through the element and the wire. If not then the wire has either come apart or broke somewhere along the way from the panel in which case you have to gain access to the water heater compartment. I've included two earlier posts showing the water heater compartment and the wiring problem that may be the same as what you are experiencing. One positive thing is the water heater is on a single feed from the panel and running a new feed wouldn't be too bad. The biggest and most time consumption is getting the water heater panel free from the water heater. It's siliconed big time behind the flange and takes multiple times of cutting with al ong-blade razor knife to get through it.
Solved- GT Electrical Nightmares