IS your current water heater propane OR electric and propane?
IF electric/propane.....operate BOTH heating sources at same time for 'fastest' recovery.
Whether yours is an Atwood or a Suburban Brand changing for 6 gallon to a 10 gallon will require cutting a larger opening (and new framing) in the side of RV.
Atwood...same width but height is 3" difference and needs an extra 2" for depth
Suburban.....roughly 4" H & W difference and at least 1" for depth.
Now if yours is propane only.......direct swap for a 6 gallon combo unit (electric/propane)
Otherwise......wet down/stop water flow---soap up/rinse off ('NAVY' shower----but 4 yrs in NAVY never took one nor do I in my RV :B )