I had to replace mine when the threading for the anode got so rusted and messed up that I couldn't re-attach the anode one Spring. I tried to repair the threads with one of those special drill bits, but it didn't work.
I replaced it with almost the same model, a Suburban Water Heater SW6DEL. This one has the option of heating using AC, as well as propane. Getting the old one out was slow and more work than expected.
Breaking the old (from 2000) seal of silicone around it was the hardest. I used an OLFA 18mm Heavy-Duty Utility Knife (using their long single blade that I bought separately, OLFA 9009 L-SOL 18mm Heavy-Duty Solid Blade, as it comes with snap-off blades) - the long 4" blade allows sliding deep into the seal so you slice through it entirely. After breaking the seal, it was still solidly stuck together, so I used plastic wedges (Katzco Plastic Doorway Wedge Kit) carefully tapped into one side at a time - inserting one wedge, then leaving that in, inserting another wedge a few inches over, and then a third. Pried the frame up bit by bit, with no scratches to the fiberglass gelcoat.
The new SW6DEL wasn't as well manufactured as I expected - the metal rim was not entirely straight, but kind of bowed in. Was a pain to get it all in and sealed up, and had to use extra butyl around where that bowing at the top was, but it worked, eventually. If you get the DEL model, you need to purchase a new inside switch for it and also AC wiring from the heater to the Distribution panel, so it is more work, but I like having hot water without using propane when I'm on shore power.
Oh, and getting my old one out was not so smooth or easy, as the tank itself had swelled over the years. I have no idea why that should happen, but it was obvious looking at the old one and the new one, that the old one was swollen in the middle. It was too fat to be pulled out through the hatch even after removing the styrofoam around it (a good 1/4" to 1/2" and fiberglass has no flex), so I had to remove it from the inside. But then the metal rim on the outside edge didn't have clearance to be pulled inside, so I had to rip that off and get it out through the hatch.
Overall, it was a much bigger job than it should have been. Makes me appreciate when a mechanic comes to me with reasons why a fix to my car or the RV took more labor than expected or usual - when stuff gets old, things happen. Though kind of glad I did it myself because I wouldn't have liked it if an RV shop tech tried to force that tank out through the hatch, and then damaged the fiberglass wall (though hopefully they would have come to the conclusion like I did to open up the inside and remove it that way).
Good luck,
Bob
The first picture shows the tank bulging in the middle. It is showing sideways, click on it for the full size and correct orientation.
This picture shows the old heater without the frame. It fits okay around the edges, with a fairly consistent gap all around. It is showing sideways, click on it for the full size and correct orientation.
This last picture shows the new heater without the frame. You can see what a large gap there is at the top.