There should be a rubber cover with two round circles on your water heater, accessed from the outside of your camper. Push those round spots. Those are the resets for the water heater.
Make sure you have water in the tank, purge all the air out by running a hot water faucet until no more air spits out.
Make sure you have not tripped any fuses in the electric panel (12 volt DC) or the breakers (120 volt AC). If they are either blown or tripped, reset them.
Make sure you have gas in your propane tanks (check the propane flow by turning on the gas stove and lighting a burner). Chances are it's fine, but still, check anyway, sometimes you just never know? Your propane tank could have leaked completely empty ... anything can happen. Just check to be sure you have propane and good pressure.
Now, fire up the water heater with the propane side. See if anything happens. If there is no clicking (electric sparking) to ignite the gas, there's a deeper issue. If it fires up, then you are good to go. Turn the gas setting back off.
Now, make sure the electric side is working. Your water heater may be double switched. There is a rocker arm switch at the water heater, accessed from the outside of the camper. This needs to be in the "on" position. In addition, you may have an electric on-off switch inside the camper for your water heater. This also needs to be "on". Flip both on and listen. If it starts heating, you will be able to hear a slight hissing, humming sound from the water heater. Things have to be very, very quiet to hear it. If you hear it, you are good to go.
Now, if you have gas working but not electric, chances are the electric heating element is blown. The are not difficult to replace if you have the correct size socket. That is the problem, finding the socket that actually fits that element. Elements can be purchased at Lowe's Home Centers or a local RV dealership parts shop.
If you still have failure with both gas and electric, then the next common part that both systems share is the thermostat. I had one replaced a year ago when I turned both gas and electric on with no water in the tank. But I took mine to an RV shop and they figured it out.
Good luck. Hope this helps.