Agree, the placard is for the MAXIMUM weight for the coach.... and a safe place to start until you get it weighed.
if you are not carrying the maximum weight for the coach,
why would you need the maximum psi?
as suggested, go to a cat scale found here
, take 10 minutes and $10 or so to get your axle weights... (loaded for a trip with full fuel tank)
then do a google search for your tires load inflation tables to see what psi is needed for your actual weights...
Also, if you substantially change the way you load the front versus the back,
you might spend another $10 to weigh again