Hat43c1981,
I do not have the same water heater as yours but I can offer some advice that may help you.
I have recently totally refurbished my 11 US gal Atwood marine water heater (1989 vintage). It was easy and cost me about $60 US (to replace with a new tank it would of cost me about $600). If your water heater is not leaking and the exterior is not a total wreak I would encourage you to do the same. most parts are easy to find either through the internet or at a local plumbing store. Most things like the relief valve, heating element, gaskets, and bronze fittings are generic and relatively inexpensive and easy to find. I decided to rebuild mine after discovering that it did not leak and, for the most part, it was in remarkably good shape for being 31 years old.
I first documented all electrical and water connections with my cell phone camera so I would know how to reinstall it properly. I then disconnected the tank and removed it from the boat so I could work on it is a better space. Make sure your boat shore power is disconnected and the electrical breaker is shut off before disconnecting the electrical wires. once out of your boat disassemble the entire tank so you can inspect all parts and ensure the tank is in good shape (no leaks, cracks, significant rust, etc.). Assuming it is in good shape then proceed to disassemble everything (pipe fitting, electrical parts, etc.) Thoroughly rinse out the tank with clean water and then finally with a vinegar water solution (there are utube videos on how to do this). clean up all the parts and fittings with wire wheels on drills and sandpaper. clean everything well with soap and water and , if necessary, paint thinner. Repaint any parts that need it (outer shell) reassemble with cleaned old parts or new parts. test to ensure that the unit does not leak then reinsulate and reinstall. I did mine over 2 to 3 days and it looks and works good as new.
It was well worth the effort and was a minimal expense.
Good luck on whatever you decide to do.
Mark
Lake of the Woods
1989 40DC
