There is a good article in FMCA mag this month on tires. The mfgr should be calling the shots on this, w/a recommended pressure for the model & loading. W/a coach of this type, you can add weight only in certain places, and most of the addable weight is in tanks. So the coach mfgr is in a position to calcualate the theoretical tire pressure to maintain the optimal tire contact patch for handling w/out having an underinflated tire.
Until you can get your hands on that info, the tire max pressure minus 5 or 10# is probably a good default, but you will probably have a stiffer ride & somewhat worse steering response due to overinflation. If the tires are old, this is not a good idea (old is 6+ years or w/visible sidewall cracking).
My front tire max pressure is 115#, and I run them between 85 and 90 based on an actual scale weighing of the front & rear axles. Drives great and no odd tire wear. (I don't have an Alfa, so YPMV)
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project