Hi Camper319r,
If you fill the water heater and turn it on it should not only get the water hot, but if all faucets are closed, there will be a little pressure as well.
Connect to campground/city water, turn the water heater on in GAS MODE for 30-40 minutes and observe for water leaking out the bottom of the camper. There's a temperature/pressure relief valve on the outside of the water heater. If you carefully open that valve by lifting the lever, you should get some scalding water coming out (which is why you need to be careful). And of course you should be able to run the shower for a while to test hot water capacity.
Don't do this in ELECTRIC mode. If there is a leak and no water in the water heater tank, you'll burn up the electric heating element.
An alternative would be to connect an air compressor set to 40 psi to the water inlet. It can take a little while to fill the water heater tank with compressed air. The compressor should shut off when 40 psi is reached. If it never shuts off, you may have a leak somewhere in the system.
You'll also want to keep an eye on the toilet. There's a built in anti-siphon valve that would keep water that's inside the toilet (not the bowl) from draining when you open the low-point drains. If it got cold enough, you could find damage that could flood the interior if you're not paying attention.
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Our owner-written
Winterization Guide might be helpful next time you go to winterize.
