The 5.4L is rated at 300 HP, so why all the crying that it will not do the job?
I think that the V10 is only rated about 50 HP more, and will offer a fuel penalty all the time you are not towing.
3.73 is the rear axle I would be looking for. With a 3.73, you can go up a mountain grade at 4000 RPM around 45 - 50 MPH. At that RPM, you will be making 250+ HP and keep going up the grade!
My motorhome is a 96 with a 460" V8, factory rating of only 225 HP at 3,600 RPM. The 5.4L is much better, due in part to the smaller pistons and ability to run 4,400 RPM all day long. The V10 has the same size piston, and stroke, just 2 more cylinders.
The truck has plenty of weight capacity for your intended travel trailer. But most fifth wheels will have more than 2,000 pounds of pin weight, and you already realize that is out. . .
HAve fun camping!
Fred.