1999 F150 towing 6500# 26' travel trailer question

I've owned four of those engines, although in Expeditions. One was a '97 and another was a 2000. If, if your engine has had regular routine maintenance, transmission serviced a couple times and you are not experiencing any problems, you should be alright. There's not a lot of safety margin there for go power and whoa power, so I would not go over 60. But that's just me. I'm not familiar with the geography but you can expect to lose roughly 7% horsepower per thousand feet of elevation.