Unloaded, you can probably use 5th every once in a while and won't really need the tow mode. It won't hurt anything, but you probably won't notice the difference.
Loaded up with the MG, I'd go ahead and use the tow mode and do most of the towing in 4th.
This, if it's an enclosed hauller. If it's not you won't have nearly the wind resistance and even loaded you might get away with some 5th gear pulling. If it's lugging, not letting the torque converter lock, or searching for gears too much, go to 4th.
We WILL need pics of the MG.
