Hardside or softside will tow about the same with the top down; I assume you don't want to tow with the top up! :D A minivan with 3500 lb tow rating should handle it fine. With the low profile of the popup, wind resistance (the usual big factor at highway speed) is minimized. Weight will kick in if you tow up a grade, but just slow down and take it easy.
I did have an extra tranny cooler on my '96 Windstar V6 when I towed a Starcraft popup one summer, from OK to ID and back. We even towed over the steep pass W. of Jackson Hole. As for my Highlander towing the Aliner, the HL already had extra cooling from the factory. If you have any doubts, try to monitor trans temps with a Scanguage or some other OBD device.