I have two wheel sets for my Trek set up with Schwalbe CX Compe cross tyres and Schwalbe Marathons... I am almost due to put the cross tyres on since winter is upon us.
I have towed a lot of stuff with that bike this year and the little load above wasn't that heavy... this was about 300 pounds of bikes that were donated to our shop that I towed across town.
I think that what I did was build a Portland before Trek built a Portland sans the disc brakes and with a somewhat lower gearing for things like towing big trailers.