Big country slide out issue

I took my 2010 BC down a county gravel road 9 miles to a campground and upon arrival I found the entire inside of my living and kitchen area full of dust. Now this is not light dust but enough dust to scoop into my hands. When I returned from the dreaded camping trip I double checked windows and vents and found the same mess only 3 miles down the road after leaving the campsite
I went with another couple who also have a 5th wheel with slide out and they didn't have any issues.
Can someone tell me what might be causing that and how to fix it?

Nan, dumb question??. Did you have any windows, roof vents or anything open in the coach??? One thing you must remember that anything left open with create a vacumn inside the coach. Going down a dusty road will suck dirt inside. With the slides "IN" can you see any outside lite coming through the slide seals?? OH, BTW, where the thermostat's left on for the ceiling vents??

No windows open or roof vents, thermostat for ceiling vent quit working 2 days after the warranty ran out. I double checked everything after the 1st round hoping that my return trip would be better... That didn't happen.
I thought about the slides after I posted this but couldn't see any light coming in however I did find that on the living room slide I could put my fingers into the rubber seal on one side quite a ways when it was in and not on the other. So my husband and father adjusted it....I hope that was the problem. I love this coach but have to tell you I am beginning to wonder, 1 year old in Aug and to date the heater has caught fire, ceiling fantastic fan quit, speakers on the outside don't work, front landing jacks blow fuses evertime I use them, back landing jack on left side doesn't retract all the way and kitchen grey water doesn't completely drain (I have to come home and put boards under the unit on one side to drain it). I think I'm getting bugged so to speak. If you have any advice regarding these other problems I would appreciate the help.
My husband is gone alot so I travel much of the time alone.
Thank you for responding

Nan, give Fantastic-Vent a call and explain your situation. They have been very helpful to me in the past. 800-521-0298.

Peace
Dave

Thanks will do and really appreciate the fact that you responded