It is like renting a car... You sign a rental agreement and pay the fee.
The cost varies to what you rent. Rent a trailer is cheaper than a motorhome, rent a small unit is cheaper than a long unit, and so on.
Cost varies by quality also.
Often the unit is stocked with necesseties and you only need to bring clothes, food and personal necesseties. I have seen lists before that recommend what you need to bring.
Check out cruiseamerica.com for more info. I have seen many of their class c motorhomes rented out at places we have been.
