I have to assume the dealer set up your hitch with an empty trailer. They should have demonstrated to you how to make the adjustments, but there are likely YouTube videos on this. Once you start packing, it is amazing how easy it is to toss in a 1,000 pounds or more in it. Weight distribution makes a difference. A travel trailer should present 10-15% of the gross weight on the ball. This is necessary for stability. Without this weight on the front end the trailer might seesaw on its axles possibly contributing to the bounce you’re feeling. Without scale weights it’s just a guessing game.
Start as others suggested by parking on a flat surface and switch off the auto leveling if you have it. Measure the height of your front and rear wheel wells then lower the coupler and lift the jack allowing the full weight to rest on the ball noting how much the rear square and how much the front might rise. Attach you spring bars using the jack to assist if necessary, then lift the jack again. If properly adjusted, the front and rear measurements should show a roughly equal drop from the initial measurements; a little more on the rear is acceptable. Once you have this right, adjust the ball height to get the trailer level front to rear. You may or may not have to readjust your spring bars. The Expy and trailer will look reasonably level once you step back and look at it. Once all is set, you can now switch your auto leveling back on.
When you go to the scales, go with the trailer packed as you would for a road trip, junk in the trunk and your passengers, full tank of gas, etc. Consider your travel: If you only go to RV parks with full hookups, you may routinely travel with relatively empty holding and fw tanks. If boondocking, you may go with full fw and return with full holding. Just consider that you may routinely be traveling with some water in the tanks.
When you get weighed, you will get a printout showing the individual weights for your front, rear and trailer axles and a total. You can typically get reweighs within 24 hours for a buck or 2. You’ll want to pull out of the way, unhitch and get just the Expy weighed (with same passengers). You now have real numbers to work with. You can verify that the GAWR for the front, rear and trailer aren’t exceeded nor the GCVWR. And you can see how much is on your front and rear axles with and without the trailer.
For math simplicity let’s assume your 6800 lb trailer is loaded with 1200 lbs of gear, groceries, water, etc. and now weighs 8000 lbs and has a tongue weight of 900 lbs (11% TW). The WD hitch should distribute 300 lbs each to the front, rear and trailer axles. If you end up with 250 on the front and 350 on the rear, that’s not necessarily bad as long as you’re not overloading the rear and your low beams aren’t scanning the trees.
If you find that the weight distribution is still way out, you may need to refine the adjustments and go over the scales once more. Once you’ve got this set, you shouldn’t have to do it again until you trade the Expy or the trailer or for some reason change out the hitch.
As previously suggested, replacing stock P-metric tires with LT may be beneficial also.
Concrete expressways can be problematic and most noticeable with a trailer.
http://www.thedieselstop.com/forums/f32/freeway-hop-42513/
There are numerous factors that determine whether it is a soft porposeing motion or a teeth jarring event. Freeway hop is a well known phenomena and opens lots of discussion about speed, wheelbase, trailer length, suspension mods, etc. in over 30 years of dealing with this, the only thing that has softened, but not eliminated it is an AirSafe hitch. Getting your tongue weight right for your trailer and properly adjusting your WD hitch is the most you can do.
