There are lots of reasons to need a tub. The problem with RVs it's hard to find one large enough for an adult to use.
One solution is to get a portable tub if needed for an adult. There are several styles one that folds out like a cot, inflatable tubs or a shallow inflatable for on top of a bed used for folks who have limited mobility.
I have seen a few of the new larger MH and 5th wheels with fullsized tubs including the whirlpool type. These are usually in 40 + sized models.
And one RV'er added a fullsized tub in the garage area of his toy hauler. His DW liked to take baths so he customized the toy hauler for her. They always stayed where there were hook ups. He added a tankless water heater for the tub water.