There will be a vent on the side of the camper for the heater is it's the forced air like our 59.
What doesn't work about it? They are pretty simple. If it's the blower it might be repairable. My blower motor is gumed up and does not wat to spin free. These motors are old enough that they can be dissassembled and they have replacable brushes. As long as the armature has not been burned from behing stalled it just a mater of cleaning and oiling the bronze bearings with a light machine oil like 3in1.
THe second possiblility is the motor is replaceed by a newer motor. Places like ww Grainger can help you out with that.
Now you do know that with these forced air units there is a switch near the heater that you need to select the power source. It will run on either 12v
or 110 "City" power. There will be a transformer in the cabinet with the heater.
Before use I would recomend dissassembing the unit and checking the heat exchanger for cracks as well as critter homes.
As for the vent. maybe the seal is glued it shut. The screen can be removed from the inside. Take a putty knife and see if you can free it. On our the foam seal stays with the fiberglass part. If the foam gives up for ghoast you can find a suitable replacement at most home stores like Home Depot or Ace.
The Hehr lifts are available. I believe Inland RV has them. It is recomended that you crack a vent or window. Most here will advice you to purches a battery powered CO alarm as well as a propane dector alarm for safety.