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!
I make it renter responsibility. I offer it as an add-on if it's not returned empty. I also provide instruction on how to dump it. My fee is $75 upon return. Keep in mind if your make this owner responsibility longer trips will likely require multiple mid trip dumps and they could be hundreds of miles from you. This alone almost makes it a requirement that it be a renter responsibility.