First make sure the water heater has a drain plug installed. There will probably be a door over the tank on the outside of the motorhome. The plug will be at the bottom of the tank.
There may be a valve in the plumbing bay, labeled 'water heater bypass'. If so, move it to the other position. If not, look where the water pipes enter and leave the heater tank (usually from the back). There should be a door or panel which allows access, but not necesarily easily There should be a 'shunt' between the input and output pipe with 1 or 2 valves to bypass the heater when winterized. Turn that valve or valves to the other direction.
Make sure you never apply heat to the tank when it is empty.
Knowing the make and year of the RV would help. On the class c there will probably be a bypass located on the inside behind a panel of the Mh where the cold water enters the water heater. If all else fails and you cannot locate the bypass call the previous owner. Welcome to the forum