I had a 1997 F150 with the 5.4 and 3.55 in a 2wd supercab... I was not towing a fiver but I was pulling a 7,700 GVWR TT and we have some pretty decent hills. Often times I would pull the hill at 4500 RPM. I never had a problem and the truck never missed a beat... but it did slow on hills.
Here is the problem with running high RPM... the truck generally has higher tolerances for RPM than the driver does. The Modular V8's and V10's LOVE LOVE LOVE High RPM's... they were designed for this and will live at 4000 plus RPM all day long.
Generally, it is us.. the operators, not use to high RPM that have an aversion to high RPM. If you can get past that yourself, you will be much happier. You are NOT killing your truck. As long as all the temps are okay, your truck is fine.
My recommendation... based on experience... is to save for a new truck with more power and more gears and simply get use to what you have.
Of course, if your overweight... performance will be impacted...and we can have that discussion as well.
How much does your Fiver weigh?
Thanks!
Jeremiah