The answer, Hatbox, is: "it depends".
How big a trailer do you need? (# of people)
How many days are you going to rent it? (most vendors use a daily rate for short stays then transition to weekly rates for longer)
Is the rental trailer available for the dates you want it for? (or is it already rented)
Do you have a campsite already reserved?
Does the vendor charge for pick-up/delivery on shorter rentals but waive it for longer?
Will you want to pay for any add-ons or extras?
What is the reputation/rating of the vendor? (you might get what you pay for)
featherface,
If you use the "Search Forums" link for the DIS and use the key words "camper", "trailer", "rental", "rent", "recommend" in any various forms/combinations directed to the Camping Forum, you'll find at least a half dozen threads from the past year with names, comments, and reviews. This topic comes up often so there is some good info for you out there.
For the record, Disney has a few approved rv/trailer rental vendors:
https://disneyworld.disney.go.com/resorts/campsites-at-fort-wilderness-resort/services-amenities/
Go down to the RV rentals and click "Read more".
The further out in advance that you can plan and coordinate both the campsite reservation and the camper rental, the better. Good luck to you both.
Bama Ed