Actually, there are three versions: The H, the S (no designator) and the L
There is a comparison chart on the brochure from ME.
Here's the link to the brochure:
https://www.marshallexcelsior.com/si...%20REV%20D.pdf
Any of the three would work on a "basic RV" with one furnace, standard water heater and no outside quick disconnect propane hookups. For most RV's with a 35K BTU furnace, standard water heater and one QD hookup, the S would likely be OK but for an RV with 2 furnaces, a tankless water heater and multiple QD hookups, the H is "pretty much a necessity"....
If it were me and the price was similar, I'd always opt for the best output available. Sort of like opting for 30 pound propane tanks rather than 20 pounders or a 15K air conditioner in place of a 13.5K unit.
Here's the brochure that's at the link above.