I see no responses to you so far, and I note your coach is a 1994 model. If its the original water heater, it might be time for a good cleaning of the unit?
Just to clarify your description, you said now the water heater switch lamp does not go out, but the heater cycles occasionally. Does that mean you leave the water heater switch on and it fails to light.......but after some time it tries to re-start and succeeds.......but does not run long enough to heat the water properly?
Can you manually start the water heater now at all and if yes, does the lamp go off when the water heater is working? Both the thermo and the ECO elements need to be closed to enable the control board to start the heating cycle. Can you hear how far into the cycle it gets (sparking, but no start; no sparking at all; smell of gas; if there is a red lamp on the water heater control board, does it light up at all when you call for hot water startup? ). Need a bit more information about how far you get into the call for hot water cycle.
If it turns out to be either the thermo or ECO, I believe that both the elements can be purchased in pairs for Atwood units, just be sure you get the correct temp values (but not sure about 1994 heater parts being the same as more modern tank water heaters though). There are videos on YouTube describing how to change both the elements....and troubleshooting water heaters.....again though not sure if the videos will apply to your 1994 coach.