Your truck is very similar to mine, except you have 3.55 gears (I have 3.73s) and you have about 100 lbs more payload than I do. My trailer is 5500 lbs dry with full propane and battery. Gross is 7500 lbs. It is around 6500-7000 lbs loaded depending on how much water I have in it. Trailer is a 24' model which is 27' bumper to ball. It's a high riding trailer with springs sitting above the axles. Truck pulls the weight well. Power is very adequate and it has pulled a few mild grades, plus one 10 mile 6% grade hill at 60 mph in 3rd gear at around 3500 RPM in 95 degree heat without breaking a sweat. Cruises at 63 in 5th gear (I lock out 6th) at ~2000 RPM and I get around 10 MPG. The engine sounds great when it's working, but it settles down and is very calm and relaxing cruising on the flats.
I wish for a bit more lateral stability sometimes in gusty conditions. I've tweaked the hitch settings to restore a bit more than 50% of the weight back on the front axles than Ford recommends and its gotten better. Wind blows it around a bit, but I never get sway...it moves as one unit. Passing trucks don't blow it all over the road, but there is some push. I think a lot of this has to do with the mushy P metric tires and 70s Cadilac style floaty shocks on these trucks. I'll be installing some Bilstein shocks in the next few weeks, and plan to switch to LT tires next spring after these Ps wear a bit more.
If I were you I'd make sure you have a good hitch setup (Equal-i-zer, Reese Dual Cam, or better). I'd keep loaded weight below 7000 lbs, but you could probably go a bit longer on length if you got a lower height lightweight style trailer.
