You can call and ask!
I have an 02 Ultra Supreme, when I call and give the serial number - they are ready to provide answers (providing they still have them.
They have been very supportive to my questions.
My unit had a SW6D (6 gallon) water heater. (which I got from a label on the actual water heater)
I just replaced it with a SW6DE.
This unit hasan electric element that can be used on "shore power", saving LP gas. Not to mention it has no pilot light (battery power will fire it off when needed)
I got mine on Amazon.
It does not come with a mounting ring, so do not remove/discard parts until completed.
Same number of gallons = same physical size = direct replacement..
NOTE: Self install will void the warranty.
It went in easy, all connections fit well. A couple of wiresneed cut and spliced to the new unit. (all colors were the same)
The "S" in the name is for Suburban the manufacturer. They have a support number as well and they were mhelpful.
My heater is a Suburban (SF_35) Suburban Furnace 3500 BTU)
Simple naming convention makes support easier.
Hopefully this will help.