New 5th wheel, what's the best tire to put on it.

You won't need a G rated tire for your 5th wheel. A good LT tire is your best bet IMO. If you have to upgrade to 16" rims for an LT, you may want to look at Maxxis ST tires. They have more weight and are a better ST than the others, according to popular opinion. Sailun is another that is getting some attention. These are all options untested by me, personally. Just giving you the information that I've gathered recently as I am in the tire buying mode right now, too.