Generally speaking, the 120 volt water heater is on a circuit alone. The 120 volt refrig heater element is hard wired or plugged into a multipurpose circuit.
A loose connection either hot wire or neutral may be the problem. Vibration while driving tends to cause that.
Certainly as suggested above, the outlet that was removed may have disconnected a string of outlets. Circuits are often wired from one outlet to the next in a string.
Sometime owners remove a faulty GFI outlet and bypass it using wire nuts to connect the remaining outlets. Wire nuts tend to loosen when there is vibration. Crimped connectors are more reliable.
Some motor home systems are more complicated. If yours is, there are many other possibilities.
I wish you good luck and happy trails ahead!