You can either buy or make a soapy solution for checking for propane leaks. To make your own, use 1/3 liquid dish detergent to 2/3 water. Use a soft paint brush to apply the solution around fittings or anywhere a leak could be possible. If you see bubbles forming, you have found a leak.
Hook up your tanks filled with propane and open both valves. Then wait a few minutes and see if you smell propane either outside the trailer or inside the trailer. If you smell it inside, be real careful....turn off the propane and let it air out before doing anything that could ignite the propane vapor!! Just by smelling it, you could get an idea where to look.
If using the soapy solution, I would start with the flexible hoses that connect to the regulator and the propane tanks. With age, they will crack and leak. From there, check where the flexible hose connects to the gas line that runs under your trailer. And then it would be a matter of checking where a gas line connects to your stove, water heater, oven and furnace.
By any chance, did you accidently leave a stove burner on? The smell of the gas could have dissipated before you returned to the camper? I assume you left the water heater on by mistake.
Whatever you do, just be careful to follow safety procedures as you look for the leak and repair as necessary. If you don't feel confident in doing the work, it's a case of where you really should have a rv shop do the work. Unless you have a bad gas line (the main one under the trailer), any kind of repair shouldn't be that bad if it is just a flexible hose or a loose fitting.
I'm just a "do it yourself" trailer owner, so take my advice for what it is worth which is about 5 cents! Above all, be safe!