Don't be afraid to REV the engine... It's peak power is probably in the 5000 rpm range.. If you are pulling 3500 rpms going up a grade doing 55 mph, what's wrong with that??
That's what my 97 F150 with the 5.4 in it did and that was an EASY 55 mph going up a 6% grade... My TT was a bit lighter than your proposed trailer, but that truck had less power than yours does too...
Anyway, don't fear the rpms... ;) They are your friend.. I had over 255,000 miles on that old 5.4 and it was still running strong when I traded it..
Like I said in my first post.. Get the proper hitch setup, load it properly and with a little experience under your belt, it'll be fine...
Mitch