Not sure. RV Rental insurance is usually outsourced to a 3rd party company (like Liberty Mutual) that may have their own policies therefore you need to be extra careful.
This is a grey area where you will need to check with your RV rental company. Our best guess is as follows for each of the rental companies:
CruiseAmerica – Their call center or web site will likely extend your reservation and also extend the reservation.
Outdoorsy – Though there is no insurance cost, nevertheless it is best to reach out to their help desk to get the rental extended, this should likely extend the insurance.
RVShare – Since RVShare charges separately for insurance for each rental day, you should check with their helpdesk to get the rental extended and also pay for the insurance for the extra day.
Note – Perhaps the likelihood of any incident happening during that additional day is low, however consider the pros and cons. If you do not spend this additional 15-20 minutes and avoid paying about $30 (for RvShare) and if any accident happens you it may cost thousands of dollars!