If you plan to rent an RV for 3 months you just as well buy a good used RV for the cost.
Renting an RV for one month could cost as much as $50.00 per day which translates to $1,500.00 per month or over 3 months may cost $4,500.00
Some RV rental companies may charge you less for 3 months due to a discount but it's not guaranteed.
It's about the cost of staying in a motel the same amount of time but you can sometimes find good used RV's for around $5,000.00 and then you can either keep it or sell it and get your money back.