Second, chill out. It's only scary the first go around. After that it's old hat.
Third, don't worry, we've got you.
The water heater fills when you turn the water pump on or connect a hose to the outside connection. Turn on the hot faucet, you're running water through the heater.
Now, if your trailer was winterized, then the heater could be in bypass. Depending on your exact design it may be different from this pic, but it will make it clearer I hope. You want your heater in normal use.
That rumble motor noise was actually the propane burner running and no, you didn't hurt anything. Once you confirm the heater is in normal mode, open the hot water faucet and make sure there's no air, if there's air just leave it open. It only takes 20 seconds of solid water to confirm this. Now, flip the switch on again. Give it about ten minutes and open the hot water faucet. It won't be max hot then, but you should have warm water. Depending on the heater size it usually takes about 30 minutes to fully heat and shut off automatically. Did they mention if it has electric heating also? If the water heater has electric also make sure not to run that while the tank is dry as that will burn the element up, but on propane there's not much to get hurt in the time you ran it.