Look in trailer life magazine online in the tow ratings, this has a good selection of trans, engine options . And lots of years. I figure to leave 10% less capacity on vehicle to give some slack while going up hills.
Just looked and it says 7800 lbs for tow rating. But that is with empty truck. So add around 700-1000 lbs of people and stuff in truck so your down to 6800. But add more stuff in trailer and I bet you would be over.
I did a test on our fifth wheel. Between empty no stuff and nothing in truck including driver. We added over 2000 lbs when ready to leave on a trip. I checked this on a certified scale which is 1/2 mile from here.