If you booked through CruiseAmerica or from any local RV dealer then they will most certainly charge you extra for late returns, in some cases the fee may be $100/hour of late return.
However if you rent from Outdoorsy or RVShare then you are dealing with RV Owners who may not be corrupted yet and may be more amenable to late return.
Ideally try to negotiate this prior to booking and have the revised return time documented in your contract document, if you have already booked then you can still negotiate and have an electronic trail.
