I have a 2009 31 SS Greyhawk I bought mine used and it was a handful to drive untill I adjusted the rear Helwig springs, new stabilizer bars front and rear, new shocks and steering damper replaced, now you can drive with one hand not two white knuckled.and the Michelin's on it were falling apart at 24k.
Our pickup would do that when we had a heavy camper in the bed...
I'd say you're riding heavy in the back of the rig, and higher speeds are 'lifting' the front a bit, which disconnects your front tires a little from the road.
If re-packing doesn't help getting more weight up front, you might want to look at some other leveling support... but I'm not sure what all that would entail (air bags would do it for the pickup).