Trailer Feels Heavier After Hauling for Period of Time

I think you might be a bit over your truck GVWR and tow rating. Have you ever been on a scale?

Not sure the gear ratio in that vehicle but a set of 4.10 would go a long way and help keep the rpm up.

As for the trailer... when was the last time the brakes were inspected and adjusted? Do the trailer brakes pull hard or even lock up when you manually apply full brakes? Do the trailer brakes seem to fully release when the manual lever is released?