How long and wide can it be? There is nothing really unique about an RV tub except that it has to fit in the available space. Here are some fairly deep models in sizes intended for the limited space in most RVs, but if you have more room you can use any residential size tub. You might have to raise it a bit to provide space for the drain, though.
http://www.dyersonline.com/rv-plumbing/showers-tubs/bath-tubs.html
http://www.icondirect.com/rv-bath-tub-right-hand-drain-tu600rh/?gclid=CM3mpvr91LgCFUyk4AodT1cAYQ
You might also consider one of this type:
http://www.tubking.com/walk_in_tubs.html?gclid=CMmIl7b_1LgCFbF9OgodexcA_Q
I think you may find even the deeper tub disappointing, simply due to lack of length to stretch out in. And the other considerations cannot be ignored either, e.g. sufficient hot water, available waste water drainage space (sewer or holding tank), weight and stress of a large tub of water, etc.
