Trailer Feels Heavier After Hauling for Period of Time

Maybe your over your payload limit and as things start to heat up, like the transmission and brakes, they don't work so well. Max tow number is meaningless. Suggest you take your loaded TT to a CAT Scale - google it.

Although your last statement seems to answer your own question - "I have also noticed a smell of what seems to be hot rubber coming from the wheels, but I had always chalked it up to the tires being warm from the weather."