I had the same issue on our 2018 Quantum RC25. When i looked at it the rubber floor mat of the cab had sell adhesive tape on it that was designed such that the mat could be stuck to the front edge of the RV Floor. Once I did this the road noise was significantly reduced especially under acceleration. I also added a strip of automotive sound deadening in the gap.
Yes the noise was there on the test drive. No excuse.....
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