The frames on the P30 chassis are somewhere between 3/16" and 1/4" thick. I'm sure you'll find at a minimum a lot of surface rust. If I were you I would certainly try using your ice pick to see if you can find any serious rust. I wouldn't be to concerned with light pitting on the frame and for sure it will be covered with surface rust.
Bottom line is you're just going to have to look at the frame and make a decision based on what you see. Personally I wouldn't want to get into frame repair on a motorhome chassis.
Good luck and let us know what you find.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)