Although we're Class A folks, we routinely have problems with getting bottom sheets to fit over deep mattresses with a topper. The previous owners of our rig put a bigger plywood base on the bed frame to allow use of a standard residential queen-size mattress.
We've found that even sheets specifically identified as being for deep mattresses are still a problem when you add a topper. A few years back, we got a 1.5" topper from Costco that we use on the S&B bed and that's a struggle to get the fitted sheet on the bed. Not sure if there are sheets out there that recognize the need to allow for a topper.
We've checked various catalogs and it seems that the only outfit still selling 1.5" thick toppers is Camping World. I guess we'll be shopping there soon.
Frank and Eileen
No longer RVers or FR owners