Ray,
To make the job a little easier on you, you can pull the reefer out of the cabinet a little bit. It's pretty much a matter of removing the "eyebrow panel, two screws under it and the bottom panel and two screws there, then open the outside compartment, remove 2 screws and the propane line. Then you can pull the reefer out of the cabinet about 6 or 8 inches. That'll give you more than enough room to work on the flue and replace the electric heater without risking cutting your hands. There isn't any extra room back there as you well know, and pulling the reefer out of the cabinet a few inches isn't a hard job. Actually, on our Holiday Rambler, it was faster to pull it out a bit than it would have been to try to change the heater with the reefer in place.
Remember, a trip to the ER to have your hand sutured is a lot more expensive and time consuming than making enough room to do the job safely
Good luck, hope it goes well for you.