I have a 2003 GMC Envoy, trans slipping that I have been towing

Low fluid can cause slipping. (depends upon how low) Most manufacturers post "safe" tow speeds. These are generally ignored. (not good). I recommend towing with driveshafts removed and where driveshaft has a "slip yoke" you install a yoke from a scrapyard and wire it to the tailshaft so it stays in place (keeps oil in and dirt out) Although you can't just detach the tow-bar and drive, it eliminates any possibility of trans damage, since the transmission cannot turn. (most use oil from pump to keep gearsets cool...when towing, pump is not pumping and over time and distance it can damage trans internals.)
Hope your problem is just low fluid and not a rebuild...keep in mind what I said though.

I also have a 2003 GMC Envoy and towed it behind our motorhome. We also experienced the same issue .... slipping with the transmission. It turned out we 'burned' out the transmission because of the way it was towed. According to our mechanic, we needed to either have a lube pump or driveshaft disconnect installed. I was told that 2004 models and higher have a N selection for the 2HI, 4Hi, 4AWD 4Low crank case whereas our 2003 do not.