Take a look at both corners of the dash while you're going over so bumps or uneven road at slow speed. See if the side of the coach is moving relative to the dash.
Some Workhorse chassis have an issue under the hood near the sides of the coach. There is a steel frame/bracket with small sheet metal screws that attach that metal frame to the side of the coach. The original screws are inadequate and break off, leaving the coach free to travel an inch or two relative to the dash, and this will cause a squeak where the dash upholstery meets the sides of the coach.
If this is your issue, it's not a difficult fix. You have to jack up the coach until the metal frame (made out of 1 X 1 steel stock) is lined up with it's original position and then install more/larger screws.
Jim