Check the rental cost. They might give a great monthly rate, but at $35 a day, it can get really expensive!
Check craigslist or other places, and see if you can buy a used one, or rent one from a seller looking to sell one. Perhaps if they are asking $1,000 for a nice tow dolly with brakes, they might rent it for say $200 and then are able to sell it later at a reduced price?
Anyway brakes are not required while towing a 3,500 pound load behind a 18,000 pound motorhome. Just downshift, and use the engine to provide some of the braking. . .
GOod luck,
Fred.