Seriously, watch the towmax tires for tread seperation before and after every trip. I purchased my 5r new with Towmax on it and by the fourth short trip one of the tires was coming apart. I've been using Maxxis since with no problems.
I know your concern was with tire pressure, and that you've already heard the Towmax are junk. But damage to your RV from a tire seperating can sure make for a bad day, and possibly a danger to you and your family.
I agree that your tires should be ran at Max pressure as stated on the tire. I upgraded from a D rated tire to an E rated tire since I had the 5200lb axles with the 6 lug wheels. I run them with the 80lbs stated Max pressure.