The dealership is much more likely to let you test drive a class C or class A motorhome than to test tow a fifth wheel.
Yes they do test drives all the time. Take it on the freeway, and see how much get up and go it has. . .
But before making a decision, look at many of them. Collect brochures, decide on a workable floorplan! Research the cargo rating, cargo capacity, and what the GVWR is. If you plan on towing something like a cargo trailer, make sure that it can handle both the hitch weight and vehicle weight. Or if you plan to tow a car, then it just need to have a hitch rating over your vehicle's weight. Yet if the motorhome is only rated at 3,500 pounds (many are from the 90's) you can not just change the hitch. It is the frame attachment that limits the weight on the frame and hitch.
Good luck,
Fred.