I've never seen a manufacturer-recommended "maximum speed while towing" for the Windstar/Freestar. Note that some states mandate a maximum towing speed that is less than highway markings. However, I just spent Memorial Day weekend camping in a popup towed by my Windstar; it keeps up with traffic just fine!
I did an informal campground survey of non-truck RV tow vehicles, and also saw popups towed by Grand Caravans and Kia Sedonas (All Sedonas are rated to tow 3500!). I even saw a short (8-foot box) Viking popup paired with a PT Cruiser (note that the PT Cruiser is based on the Neon - and is only available with a 4 cylinder engine).
It's fairly common to see Windstar/Freestar models towing mid-size hybrid campers - in Canada, where taxes and culture create different sensibilities. Curious about the culture? This article looks at why Canadians choose vehicles differently than Americans:
http://www.thetruthaboutcars.com/con...6081718401479/
For me, practical realities and budget led me to investigate towing with my Windstar. I learned a lot - and prepared my trailer and van - before proceeding. But the combination served us well in 2000+ miles of towing in the past 6 months. I suspect more American RVers will learn about the capabilities of modern minivans as life with $3+ a gallon gas becomes normal.
My frustration? Manufacturers appear to artificially hold back on the tow ratings of minivans, likely in part to boost SUV and truck sales. I'm waiting for the first manufacturer to realistically rate their minivan tow capacities - and include accurate, complete owner's manual information about towing equipment (i.e., trailer brakes and controllers; weight distributing hitches) and practices (i.e., proper load balancing, axle weights, safe towing technique).