Emptying holding tanks in a boat is a major hassel. No problem for the RVer. Everyone knows that the stuff flows downhill. Easy. With a boat special equipment is needed dockside to suck it out of the holding tank.
Of course if you are a certain distance offshore you can simply pump it out yourself. Everything else living poops into the ocean.
Foot for foot, lots of room in a RV. All they are basically is a square box with wheels. Not so with a boat, especially a sailboat. Boat designers however make much better use of interior spaces than RV builders. So much wasted dead space in RVs.