As per the rental agreement, the guest 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 hosts do offer an add-on that allows the guest to pre-pay for the emptying of the septic tanks. This would mean that the guest would not have to empty the tank themselves before returning the RV and the host would complete this for them after the return.
This add-on will be listed as Emptying Septic Tank Prepayment in the host's listing.
Some hosts may also offer this service as part of their rental fee and will not charge extra. This would usually be noted in their listing description.
To make sure you and the host are on the same page, the best practice is to ask them directly once you've sent them a booking request.
As part of the departure and return inspections, the tank levels will be noted to confirm whether they are empty upon the guest's return.
In the event that the guest did not select the pre-payment add-on and did not empty the tank before returning the RV, the host will be able to charge them during the closeout process.