I agree with the comment that if you could get the compressor repaired that you would then have an old unit with a new compressor so the probability of part failure would still be high, just not on the compressor. The cost factor is also a story I've heard many times. I've replaced both HP units on our 2003 Winnebago Brave. Both are the same model. One was done by a RV shop and cost was over $1,000. When the other went out I investigated, as you are doing, the cost of compressor replacement.
The compressor is available and it is the Tecumseh RGBB515BAA. That is the make/model number right out of the Airxcel parts manual, page 3 item 4. The Tecumseh site also has a "buy from" retail locator and I found two shops within 50 miles. However, even though you probably can get the compressor, the cost for the part and installation will be high. The unit will have to be removed from the roof. No big deal, I've done that, but at about 100#, it takes time, too people, and a block and tackle. A torch will be used during the repair so it has to be removed for safety reasons. Mechanically it is a simple task to cut the coolant lines and solder in the new compressor but technically it is extremely difficult. If even the most miniscule metal shaving gets into the line, the compressor will be ruined in a short time. Thus the high labor cost to remove and replace. As others have said, the cost approaches that of a new unit. Replacing the unit is a snap. Only four bolts to remove then lift and set down on the shop floor. If you are using the same make and model then the "lower unit" contactor box can be used.
I found a new unit online at: http://www.my12voltstore.com/RVAirConditioner_s/197.htm where you will find the price very reasonable, about $560. Shipping for my unit was a little more then $50. It came on a semi and was easily loaded into my Honda Element.
A word of caution though, the roof seal is on the bottom and the shipping crate protects it. Do not remove the crating until you are ready to sling it to the roof. Compressors are not inexpensive. I believe that I would have paid more to buy the compressor and have it installed than I did to buy a complely new unit and install it myself.