Just bought my first RV a week ago. No idea what I am doing!

16.5 tires are not the easiest to find. If you haven't, and they have any real age on them, get a new set. Better than trying to find one on the road somewhere or having to wait a day or two while one is ordered in.

Edit: I see where you say the tires are brand new. Is it just that they look new? (and could be several years old) or were they just installed, and have you actually checked the date codes? Even brand new tires can turn out to be a couple of year old or more.

Charles