Do you have a small silver exhaust port right behind where the sink should be that's NOT connected to the water heater?
We have a 1994 Starcraft Starburst popup with a 12' "tub", which I'm going to guess is somewhat similar. The sink goes on the drivers side, right in front of the bench with the freshwater tank. The furnace goes under the sink near the floor (inside the sink cabinet) and exhausts through the side right behind the sink, thermostat is attached to the left side of the countertop with the sink. It will happily drain an entire 20 pound barbecue tank in one night if it gets below freezing. You can probably find a damaged popup with the heater in it pretty cheap, just make sure it hasn't already been stripped for parts first.
I'm going to take a wild guess that this is the missing heater:
Suburban SF-FQ Series ducted heating furnace capacities range from 20,000 to 40,000 BTU/h. Small, compact and powerful with unlimited design and installation flexibility.
suburbanrv.com
It's over $1000 new, so finding a wrecked or damaged camper that has the same style would probably be your best bet. This style just has a little outlet on the outside for the heater exhaust, the other style has a full cabinet on the outside with a door you can open to service it.
Our didn't have a water heater at all or pressurized water, just a manual pump on the sink so its possible you just have a water heater instead of a furnace, but those furnaces are tiny you could easily shove one in somewhere.
I was also thinking even if yours never had a heater to start with, the other side of the camper from the sink has the propane line that goes to the external bracket for the camp stove, you could probably put a used furnace there