Had the same thing on my '01 Ford Chassis, Ford change out the suspension bushings. Not sure which ones, but there was TSB out for it. I was in for Transmission work. The tech did a search and found that other things to look at. Found a TSB on the vacuum system that would shut of the ventilation system on a slight incline. He also detected a leak from the Power Steering pump and replaced that. He was waiting for transmission parts so he had time.
That was @22,000 miles in 2004 (I think).
So, Transmission fixed (seals), A/C fixed (canister, I think), bumping under seat fixed (bushings), and PS pump replaced. All under warranty! Never saw any paper work, thus, not sure what the parts were. Front office was closed when I picked up the rig, and just drove away! The only Chassis problems I've had in ten years! (other than normal stuff)
With all that said, heard some knocking under the copilot seat a year or so ago! That was a worn out shock @ 60K miles, replaced all four (DIY, Monroe Gas Magnum RV, or, something like that). The shock was more of a banging sound and louder on a rough road, not related to turning. The bushing was more of a "thunking" sound and not related to road condition!
Good luck!
H
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'01 National RV Tropi-Cal, Ford V10, '01 Suzuki GV 4X4 Blue Ox Tow Bar,300 Watts Solar, 2500 Watt '458' Inverter, NO TVs