As per the rental agreement, the renter is expected to bring the RV back in the same condition as when they collected it, and this includes the black and grey water tank levels.
Some listings do not impose on the renter to empty the tanks, as the owner would offer this service as part of their rental fee. Other listings offer an Add-On for an extra fee for this responsibility to be taken care of by the owner.
If the renter selects the Add-On, it means the owner will dump the tanks after the RV returns.
If the Add-On was not selected and if the owner is not expected to empty the tanks, then the tank levels should be noted when picking up and returning the RV as part of the inspection form.
Those readings will inform the owner whether they should charge the renter or not at their return; meaning if a renter collects the RV with empty tanks and bring them back full, the owner will charge them for this service during the closeout at the rate highlighted in the listing.
Note that charging for emptying the tanks is subject to the service fee and tax.
Make sure to double-check with the owner about this topic when you are discussing the rental details.
If you empty the tanks and get a receipt, keep it on hand as it might be useful if the readings are incorrect.
Always make sure to use the special RV toilet paper and to never discard any item, tissues, paper towels, baby wipes, and fabrics or rags in the toilet.