If they are towing/driving themselves yes they should. If I do delivery, the emptying the tanks is part of my delivery fee. I don’t feel like they should be charged extra for that, because if they have stationary insurance, they can’t move the rig anyways - or shouldn’t be. If the tow/drive it themselves and it comes back not dumped, I charge a $150 fee to have to dump it.
There is no "one-way" for this. Say you'd like to offer the dump service as part of your rental - OK. You prefer the RV is always dumped - OK.
The big thing is making sure your expectations are clear to the renter and they agree. And if you want it dumped and they don't do it - make it NOT worth their while.
YMMV - safe travels!