Assuming this is a gas water heater, the problem could be several things. Check to see if the igniter is trying to light the flame when the heater is on. You should be able to hear the clicking if it does so.
It may also be the pilot light if your water heater uses one of these old ignition systems. By using compressed air to clean the pilot light supply line you may solve the problem.
It is possible the thermostat may be faulty and is not allowing the system to activate. Replacement of this part may solve the problem of the unit not activating.
Since the camper sat up for so long, I would recommend using compressed air to blow out the gas line leading to the water heater.
There may be a blockage in the line caused by water or some other substance which prevents the gas from reaching the water heater. You should be able to check to see if the gas is actually getting to the unit and then take it from there.
Feel free to ask again if these tips do not solve your problem.