Welcome Kerry,
We can definitely help you figure out whatever is causing the trouble. Suburban and Atwood are two common water heaters (WH) on RV's and it would help us to know which you have in. Otherwise we might be running you in circles troubleshooting a part that doesn't even exist.
Open the outside door to the water heater and see if there is a name and model number. If no name, try looking at the drain plug. If it's plastic (nylon) you have an Atwood, It it's metal you most likely have a Suburban.
If you can give us the year make and model of the RV, we might be able to deduce what WH you have.
As far as the actual problem, many issues are the same across the brands.Can you describe everything that happens and any switches you are flipping? Do you hear any clicking when you turn the WH on? If so how many clicks and if they repeat, how many times do they repeat? Is there a whoosing or popping sound when it tries to light? Is there a trail of black soot up the side of the RV above the WH? If you flip the Temperature Pressure Relief valve does water spray out? Do you have enough propane? Do the burners on the stove top light?