The porpoising seems to happen when the speed is anywhere between 25-50, but it seems worse when the speed is 35-40 MPH.
All mechanical assemblies have a "resonant frequency". That is a frequency when bad things become worse. My RV's front end bouncing up and down is a frequency and 35-40 MPH seems to be its resonant point.
I called my Dealer (in Alvarado) today and the service manager had never heard of this problem and they sell a LOT of these 28QBs.
Because they do not do work related to the chassis or drive train, he referred me to a Chevy Truck dealer in Ft. Worth. The RV service specialist there had not heard of this problem either, but commented that my Chevy 4500 chassis probably was not strong enough for a 30 foot (bumper-bumper) RV.
She did suggest installing heavier shocks.
I have a good friend who had a similar problem with his 31 foot Winnebage. Last weekend, he installed Heavy Duty Bilstein shocks. Today, he told me that eliminated the problem.
Changing the shocks obviously changed the resonance frequency.
I'm going to order H D Bilsteins and install them on my 28 QB.
We've got a 2,000 mile trip planned for next week, so I want to do it quickly.