On our 19-foot Bambi, the hot water heater switch is under the bathroom vanity mirror, behind the water faucet handles. It is an OFF/ON switch with a red light. I'm not sure where it is on a 30-foot model, but it sounds like you may have already found it.
See link below for instructions on how to purge air from the propane lines and light the water heater and refrigerator (if you are having difficulty with ignition). By the way, make sure the propane valve is OPEN, and the tank(s) you are using are at least 1/3 full.
To clarify, the red light originally comes on when attempting to start. Then, if the water heater successfully lights, the red light goes out. If the red light stays on, the water heater is NOT lit.
You can confirm attempted ignition by listening for clicking sounds outside near the hot water heater access door. When the burner ignites, you'll hear "whooshing" sounds of propane burning (like a BBQ burner on high heat).