Your bypass valves are going to be on the backside of the water heater itself.
Yes, you can winterize by filling the water heater with antifreeze, but nobody likes doing that. It'll use 6 gallons of anti freeze in the water heater alone.
It is possible that your camper did not come with a water heater bypass installed, though that is rather rare these days.
Once you find the backside of the water heater, ensure that you can see one water line going into the cold fitting (bottom) of the water heater, and one water line coming out the hot fitting (top), with no connecting line between the two lines with one or more valves in the circuit.
If you establish that you do not have a bypass kit installed, I'd recommend purchasing one and installing it yourself, then winterizing. They really aren't that hard to install.
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