Here's the deal, I'm a tech and former service advisor and work with extended warranty companies on a weekly basis. Ask the shop to show you the published labor guide times where they came up with 20 hours. I could justify a few hours of diagnostic time + whatever the published labor time is to remove and replace a transmission + there might possibly be some TCM programming to reset adaptive shifts, this may be an hour at the most. I would say 10 to 11 labor hours is probably what the job is worth. Most extended warranty companies will not pay for diagnosis or programming an ECM therefore that would be your responsibility.
FWIW, at the Dealer I work for, customers are not asked to pay any additional labor beyond what the ext warranty company has authorized, with the exception of diagnostic time.
The actual time it takes a tech to do a job is irrelevant, repair shops don't bill for labor that way. I lose my fanny to inadequate published repair times on a daily basis.
I'd have to guess the tech doesn't want to work on a motorhome, so he quoted the service advisor a huge amount of labor hoping you would go somewhere else. This ain't my first rodeo.