Yes you can lose the entire amount you paid for the reservation. E.g. if you bookd an RV that has a ‘strict’ cancellation policy (i.e. no refund if cancelled within 30 days of RV rental pick up) and try to cancel it 15 days before your pick up date then you will lose the entire amount you paid for the reservation.
You may find this baffling but this is a harsh but common business practice in RV Rental industry. You can mitigate this risk by booking with RV Owners that have better cancellation policy e.g. they will refund 50% of your amount if you do not use the rental.