If an RV battery is weak then an RV renter does not have a way to identify this at the time of RV pick-up. This is because at the time of RV pickup the RV engine or generator will temporarily charge the battery and it will show as fully charged.
However once you turn off the RV engine or generator then the RV battery will drain easily. E.g. if you are travelling from Chicago to South Carolina in March and stop at a rest stop in Kentucky and need the RV furnace to run most of the night.
In this situation a weak RV battery will not be able to run the RV furnace and will likely drain completely by morning.
While there are technical ways to measure the strength of the battery, a typical RV renter does not have the knowledge or equipment to test the RV battery strength. One way to mitigate is to inform the RV owner well in advance that the RV battery should be in good condition for the RV trip and emphasize this as teh date of RV rental ddraws closer.