Just remember that t-stat is only fr the electric heating........
Propane with its own set of t-stats will be OEM 130*F
And I'm surprised you found that 120*F optional set......Suburban doesn't offer them anymore. Must be Old Stock
Atwood offers an 'adjustable' t-stat (110*F - 150*F) but not compatible with Suburban Models
And swapping Atwood for a Suburban .........not same size opening so major mods needed. Hardly worth that.
ON EDIT:
Using your RV circuit breaker to turn water heater on/off.......
Probably should buy a space CB so you can swap it out when OEM CB fails. They are NOT designed to be used as a 'switch'
There ARE CBs with that rating but your OEM is not one.