If you are truly going to use it for only one month, my first impression would be to rent. You really don't want to buy a new RV then try to sell, you will loose $ big time. You might find a used unit, then resell, but that would mean a little work on your end both finding a good deal and then selling.
The rate to rent is going to depend entirely on what you rent, when you rent it and for how long. I think it would be safe to say that you're looking at anywhere from $500/week for a small Class-C up to $2000/week for a large diesel pusher Class-A. Any rental company worth doing business with would also include a discount (typically 10%) for multi-week rentals. For a better idea of what rates would be like, check out these sites...
Grandview probably would have a better feel for it, but I would bet you that a dealer who sold to you with a guaranteed buy back price would have to make the difference between sale price and the buy-back price far more than the cost to rent the same RV for the month that you want to use it. A dealer is no different than any other business in that he has to show a profit. If sells you an RV that is new, and then buys it back from you, he will have to sell the now used RV for a price equivilant to one that is one year old, or more. A rental business has the capitalization to carry that kind of business. No dealer can sell and buy-back at a good price and still be a profitable business. I would be very careful of such a deal!
Yea, there is no way a dealer can sell a unit and offer a price in advance for a trade-in or buy-back. For one thing, there is no way to value what could happen to it in a month. IF some dealer tried that, Kirk is right that the dealer would have to protect his interests. That is why I said not to consider a new unit and it would be very hard to do it with a used one. I think in this case, rent is the only option.