Need more info, but I will try to start you out.
If the rear bath has a low cabinet next to the toilet, you will need to disassemble the e cabinet to get to the back of the heater. If there is a closet it is in the bottom of the closet under a thin panel. It could also be under the bed if you have a bed on that side. You want to inspect the back of the heater first, it may just be a leaky fitting, but do not get your hopes up.
In the older trailers the tanks will rot from the inside out if anode rods have not been maintained.
Removal is simply disconnecting the rear lines, removing all of the screws under the edge of the outer door and disconnecting the gas line after turning off the gas. The heater should pull straight out. The new one of the same size should fit exactly. You may have to make a hole for the propane line. Save the door off your old one it may fit the new one depending on the old ones age.
Installation is the oppisite of removal.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato