Welcome!
Just some clarification....OK
You posted in 'folding trailers' but mention bought 5th wheel???
RV toilets have water in bowl to act as a 'vapor barrier' (and to keep bowl seal wet)
Residential toilets have water in bowl due to built in 'p' trap in toilet base casting.also for vapor barrier
RV sales folks typically do NOT know their products so rarely listen to them or follow their advice
Sealand toilet to Thetford.....OK but you do not NEED to go with the Thetford Aria Deluxe II toilet that requires 12V DC power to flush.
You would not only need to R&R toilets but you would need to provide that DC power ------fuse/wires/connections etc
Check your Sealand against this Thetford replacement chart....
LINKAnd whenever toilet is removed I always recommend adding a shut-off valve to toilet water supply line so that water to toilet can be isolated vs having to turn trailer water supply off.