I just talked to Keith at NL again. He said that the stall mat is actually ok in his opinion as long as it provides sufficient clearance and that I just need to add a soft rubber mat on top of it (adds some give and traction for sliding). I looked at my local lumber stores and none seem to have 1" plywood for some reason. They only stock up to 3/4 and then jump to 1-1/8" tongue and groove.
His thought on the gap in the dinette is that someone didn't have is secured properly and hit a bump causing damage. I didn't see any nor did my dealer when they inspected. If it is sagging due to some structural damage (as seen by there being a difference in basement height front to back), adding a 1" plywood sister to the basement wall should fix things.