Both have their problems. We have a '95 Chevy P30 Chassis ('96 motorhome).
P30 is notorious for problems with the Auto-Park brake. Ours is no exception.
As others have mentioned stock suspension isn't great on these; however, with the right aftermarket parts you can really make them handle very well. We've had some suspension work done at Brazzels in Centralia, WA. Shocks and swaybars made a big difference.
Bottom line I think is to pick a motorhome that you LIKE (floorplan, features, options, etc.) and then have a mechanic check out the engine/chassis. If it's in good shape, I don't think it matters much if it's a Ford or Chevy.
Of course, I'm a Chevy guy, so I wouldn't own a Ford
But that's just me.
Here are some links to mechanical work on our motorhome: