We have a MasterTow which works well (although I still want to switch to a 4 down towable). MaterTow makes a good product but there may be lots of others as good. One thing that I have noticed, though, is that an empty dolly is a pain to tow. When we have the car on the dolly, i feel absolutely no effect from the dolly beyond the obvious impact of the additional 4500 lbs back there. However, I have towed the empty dolly for a few miles and hated it. It is so light, it bounces all over and you can really feel it in the coach. Particularly when on a rough surface, it it really annoying. I was recently following another vehicle towing an empty dolly and I could actually see the dolly bouncing clear of the road on some bumps.
--jack