I have found that some of the sheets I have that were previous to the days of the super thick mattresses work well. When I bought new ones, I didn't buy the "extra deep pocket sheets." With a 3" memory foam and a quilted mattress pad, I am good with regular sheets, just have a bit extra to tuck under at the foot.
FYI, OP mentioned not being able to find high thread count sheets in "RV" sizes. You do not want to get too high of a thread count or you have defeated the purpose of cotton sheets. A count over about 400 doesn't breathe well, and you end up with the same temperature regulation issues as if you used polyester blend or 100% polyester sheets.