I just posted on the Miramar thread about tires. I have Michelins, not sure what brand you are running, but do a google search for the brand of tire and pressure chart, example "Michelin tire pressure chart". You will find a chart that breaks down by axle what PSI is recommended for your tire!
I loaded my rig with fuel, LP, food and all the junk I normally take when we head out. I rolled over a DOT truck scale and weighed both axles. I went into the chart and found I should be running my tires at 77 PSI versus 90 PSI. The latter is what Thor recommended, it appears that 90 PSI is based on the max weight of the coach, which is 22,000 lbs. Since I'm coming in around 20,900, Michelin recommends 77 PSI. Here's the link below, hope it helps!!
Michelin North America RV Load & Inflation Tables