What year Ford truck? If it's fairly new you can look at Ford's towing site. Better yet, you can also contact Ford and give them your vin number and they will tell you your trucks towing specs. Look at the travel trailers and try to keep the trailer at 80% of it's gross vehicle weight rating. You have to consider what your putting in the truck such as wife, kids, toys, tools, etc. Also if your going to haul with full tank of water, food, clothes, pots, pans........you get the picture. My 2005 Ford diesel weight rating included a full tank of gas and a 150 lb driver..........which I am not! :
Our 28' 5th wheel is 7,600 lbs dry and loaded with 59 gallons of water, all supplies, generator, etc I come in at 9,600 lbs. My GVWR is 11,500 so I am under 80% of capacity.
I have a diesel and with my load I don't worry about altitude at all. If you are sure that you are going to buy a gasser, I would not worry about the altitude because you can always shift to a lower gear when you are pulling the hill. Try not to make this too complicated or you will go nuts.
So again, find the towing specs on your truck before you buy it. Consider that 10-15% of the total loaded trailer weight will be on your trucks rear end and try to give yourself about 20% leeway. Add whatever your going to put in the bed of the truck along with the cab.
Just out of curiosity, why are you so adamant about a gas engine?
I'll send you Ford's towing site on the next post.
Good luck, Stan