I'm not sure I know what "chugging" is, or feels like. But there is a phenomenon called "chucking" that is manifested as a subtle back-and-forth or up-and-down motion of the tow vehicle. If I recall correctly, it happens when you are driving on a concrete road (not asphalt) where the center of each slab drooped a little as the concrete dried. This subtle "swooping" of the road surface can set up a rhytmic motion of your rig as you drive. Does your chugging go away when you reduce your speed, or drive on an asphalt road?
This has been discussed a few times on this board. Start with these threads, or do a Search on "chucking".
http://www.trailmanorowners.com/foru...ead.php?t=3427
http://www.trailmanorowners.com/foru...ad.php?t=13202
I hope that "chugging" does not refer to a situation in which the engine is running at very low RPM - what we used to call "lugging". An automatic transmission should prevent that, presuming your tranny is an automatic and not a manual.
Bill