There's going to be a lot of different answers to your questions because of the nebulous term "reseal". I'd suggest to make the dealer explain what they are going to do. In fact, that is a good strategy with any service being done on anything.
Good point.
I've never applied a rubber roof coating on my 20 year old 32' 5er.
I do a twice a year sealant check.......roof jacks and other fixtures check. I've never reseal the whole roof as its simply not needed.
I know what to look for when checking sealant up there. All I've ever done is add a bit of Dicor over hairline cracks in the sealant without making a dam for water to sit in. I've had a few places the old sealant started coming loose so I pull off what comes off and reseal.
This unit has never leaked which they won't when properly maintained.
Some rv owners would be better off to pay the dealer for this service
if they don't know what to look for or how to properly seal/reseal a rv.