Awful brakes

Yes, old fluid does have a reduced boiling point, but your truck's braking system should not be heating components to the point of boiling brake fluid while your driving around town at 25 - 35 mph -- regardless of pulling the small trailer. In fact, the trailer shouldn't even be a factor if the trailer brakes are working properly.
The only time brake fluid boiling should be a factor is in *extreme* conditions. Like towing a 5K+ lb trailer down a long steep grade. After you've downshifted to utilize engine braking, but the grade is enough that you still need to continually apply brakes. In that scenario, I'd be very concerned about boiling fluid and brake fade. In which case I'd pull over periodically to let things cool down.

Assuming what you are experiencing is indeed brake fade, then something is wrong with your braking system causing your brakes to overheat. That's a symptom, not the problem.

Is your brake controller set up properly for the pop-up? Have you adjusted/tested it per the instructions? Can you panic stop at low speed using just the trailer brakes?

As I said earlier, test your rear brakes after a few panic stops. If they're not adjusted properly (which is common), they're not contributing which puts the entire stopping load on the front brakes.

If both of those check out, move to the front brakes and see what's going on. With all due respect to the reputable shop you spoke with, 17 year old calipers and wheel cylinders are suspect. I do agree that uniform pad wear is an indication that things are working properly, but you can't be sure what's going on without further investigation. If your fluid is old and dark, that tells me the braking system hasn't been well maintained. Look at the pics posted by @Professional Hand Model . His calipers looked fine from the outside, but the internals were a mess caused by corrosion getting past the boots & seals, and build-up of gacky brake fluid.

 

Brake hoses were compromised, as well.

 

I will put some hawk HPS 5.0 brake pad, stainless steel lines, and ATE dot 4 fluid.

 

In your turd gen?

 

I did that on my HPDS car for NASA event. Hawk blue 9012, stainless line, and ATE fluid.