On my '71 Safari, I had to remove the exhaust cover on the outside of the
trailer. There are two screws holding the inside exhaust pipe to the
trailer wall under this cover that have to be taken out.
Then inside I removed the front cover of the heater off. There is a screw
holding the heater in place in the housing. You remove the screw, the gas
line and electrical wires. Then the internals of the heater slide out of
the housing for servicing.
The actual heater box and vents stay in place.
good luck
-Tim