I think I found the
Toyo RV inflation guide here.It would be nice to know your corner weights, but if your tires are equally loaded, it looks like you would use 90 PSI in the front and 90 in the rear tires as well. So I think you are correct.
I would definitely call Freightliner and get a good four corner alignment done. Look up your ride heights and have them adjust that as well. The rear wheel alignment can be as important as the front -- especially the way you are describing the rear going wide. While they are checking your suspension, have them check for loose connections, ball joints, etc.