There have been several posts regarding tire wear on some specific tow dollies. I towed a Demco Kar Kaddy some 30,000 miles+ with absolutely no tire wear. I've read others say they have worn tires out in a few hundred miles due to bad alignment. As I remember (scary), they had to heat the axle and bend it to obtain the correct alignment.
I don't know if alignment is your problem but thats the second place to look and you've already looked the first place...air pressure.
Jim with Judy
08 38' Bounder DP
Toads: 08 Jeep Wrangler Sport; 11 Chevy Malibu