Do truck and trailer sit level when hitched up?
If trailer rides nose up, that shifts weight off the tongue and allows wind underneath the trailer. Best would be slightly nose down.
Anything mounted on rear of trailer, or, too much cargo loaded behind the trailer axles?
This will also take away tongue weight.
Are your weight numbers somewhat acurate?
935 tongue weight is less than 10% of your 9860 gross weight. Tongue weight should be 12 - 15 percent.
Could also be wind or ruts in the road.
My fresh tank is behind the axles. When I load up with water, it takes 200 lbs off the tongue and I try to put more cargo up front to compensate for water weight.