He brought home a 2006 f150 4.6l 4x4 ...
I have a 2003 SuperCrew 4.6L V8, 3.55 axle, 4x2, tranny cooler but no oil cooler. I drag a 7x14 cargo trailer that grosses up to around 7,000 pounds. On a recent trip of 300 miles across the Rockies in west Texas (5% to 6% grades) with the trailer loaded to about 7,000 pounds my rig did just fine. Granted, it downshifted to 2nd gear = 4,500 RPM @ 65 MPH on the 6% grades, and it was constantly downshifted out of overdrive = 3,000 RPM for any little bump in the road. But it never overheated and the 4R70W automagic tranny seemed to be fine with that abuse.
And yesterday I returned from a trip to Phoenix. We hauled a load of household goods to Phoenix, then returned to Midland TX empty. The trailer grossed about 5,000 pounds going and 2,000 pounds empty. There was literally no difference in performance or MPG with the trailer loaded or empty. About 11 MPG both ways when cruising at 65 MPH. One leg from Phoenix to Tucson empty I kicked it up to 70 MPH and got only 8.8 MPG. Conclusion. Speed and frontal area kill MPG and cause a drag on the truck. Weight in the trailer doesn't seem to matter.
The Ford 2006 RV and Trailer Towing Guide says your truck with 3.55 axle has a tow rating of 6,200 pounds, but that assumes absolutely nothing in the truck but one skinny driver. So my guess that your truck should be able to handle a TT that weighs up to about 5,500 pounds with no problems. So you should look for a TT that has a GVWR of around 6,000 pounds. When traveling, have empty holding tanks and only a few gallons of fresh water in the water tank and you should be fine.
For example, here is a trailer that should suit your needs. The first one in the list at the following link - model 179QBWE. Shipping weight plus carrying capacity = 6,030 GVWR, so you should be able to keep the weight down to well below 6000 pounds with empty holding tanks. Separate shower so you don't have to sit on the pottie to take a shower. Queen-size bed without having to tear up a dinette or couch is important to me. In fact, if I bought this one, I'd probably throw away the standard mattress and replace it with a better mattress, such as a Serta Euro-top mattress available at Sam's Club (and probably at CostCo) for around $500.
Keystone Springdale | Specs
Put your cursor over the illustration and the floorplan will pop up big enough to see it.
If you'll be traveling in the desert southwest, then you'll probably want the option of the 15k BTU AC instead of the std 13.5k. (Note the options for that trailer.)
As with all RVs, this one doesn't have tires with enough weight capacity to give you a good fudge factor when loaded for bear. And bigger tires are not a factory option. So I'd replace the tires/wheels with something that has about 25% or more increase in weight capacity. Maybe include the tires/wheels as a dealer-installed option in your order. I'd go for 225/75R15E tires on 6" wide rims if they will fit in the wheelwells.