I had a porpoising problem on my 2006 Chevy based Class C also. The stock shocks are not up to the job.
Changing the air pressure will do no good at all. What is needed is a good pair of shocks to stop it.
Like a good set of Bilsteins
LIKE THESE from E Shocks. They have the best price around.
After they were installed all the porpoising went away.
Good luck with this.