"BALANCE YOUR TIRES! Tire shops do not usually balance trailer tires BUT if you don't it can shake the camper to bits"
X2 on this statement from another poster.
The cost is minimal and I am sure it will correct the problem.
I balance my trailer tires and run balance beads to compensate for the fact trailer drums if the trailer has drum brakes that are not balanced.
I have several times forgot and left items on the dinning room table. Even after a couple hundred miles I have found the items are still on the table.