[Richard and Rhonda]

12 inches is the right number. Just inside the fender wells you will find square tubing at the bottom of the fender well. Use that as your reference point.

If the shocks are shot, that will cause you to porpoise. Or if the shocks are too hard in the front you porpoise. More on that in a minute.

You can remove the bottom nut on the shock mount pretty easy. If you can move the shock easily up and down by hand then you have a shock problem.

I have Koni adjustables on mine. When I first put them on I had them set too firm. The most noticeable symptom was popoising when I hit a dip. If your front height is too high, the air bags are pumped up too hard, and it could be causing the same thing. The height control valve is in the very center of the axle if you have a beam axle. I don't know it works if you have IFS. Get your ride height ride before you do anything else.

It is possible that the adjustment slipped when they jacked the coach up for the new tires.