I have an 05 MS so I don't know if the switches are the same. I'll tell you about what it takes on my rig.
Before you do anything else, make sure there is water going to the hot water tank. There is a relief valve on the top of the hot water setup above the burner. It has a small spring loaded lever. Pull up on the lever and see if water comes out of the valve. If it does you are OK.
If no water comes out your feed line is closed off. On the inside of the basement behind the water heater there are 2 lines going to the water heater. One should have a small valve on it that shuts the water off. Open this valve and check to see if water is going in the heater again. You'll need to fill it if it was empty. Hold the spring lever open until water just starts to come out. You need some air space in the heater so don't hold it until there is a full stream, just a bit more than a splash will do.
Once the heater is full make sure the propane tank valve is open.
My heater works on either 120 volts or propane. The 120 volt switch and the propane switch are in the coat closet by the door. You will only use one or the other, not both. In my case the 120 volt switch is labeled electric water heat, just flip it on. If all you have is the propane switch look for a small switch on the board that has the tank level indicators on it. There should be at least 2 switches there. One is the water pump, the other should be the water heater. If you turn either of the switches they light up in my rig. You'll hear the pump start with one and the other may be pretty quiet.
If you hear the propane fire up you're done other than waiting for the hot water. If you don't hear it your burner or igniter might be dirty or clogged with insect nests. Go look, clean them out and try again. If there is an audible clicking and no burner check to see if the pilot light pipe has been blocked by bug activity. Clean and try again.
If you use the 120 volt heater, provided you have one, you likely will not hear anything. You'll have to go outside and listen for small noises indicating water being heated, similar to an electric coffee pot before it starts to perk.
If none of this helps I'd suggest getting it to a service center and have them look it over. The control board could be bad, wiring could be loose or a number of other things. If you tried to turn on the heater without water in it you may have burned out the heater element (electric) or over heated the burner can (propane).
This was long but I hope it helped.
Mike Nancy and the fuzzies
Fulltiming since June 2004
Volvo 660 MH tow vehicle
2005 MS 38RL
2007 Saturn Ion "toad"
2010 Gold Wing "piggyback"