The grey suede side walls down in my cabin have become rippled and started to hang down a little. Is there any kind of adhesive that you all would recommend to make that stick to the walls? Maybe some kind of "head-liner" adhesive?

It's likely the adhesive is STILL doing it's job. The problem is probably the foam backing coming apart and leaving 1/2 of itself stuck to the walls and letting the other 1/2 (which holds the part you see) fall. More adhesive is usually a temp. fix, complete replacement is usually necessary.

+1, once you fix the foam there is a 3m product called General Trim Adhesive that comes in a spray can @ auto parts stores that I've had good luck with

Is this a big project? Also, what kind of foam do you use?

I did mine last year, hardest part was removing old foam. I used new 3/4 foam you can either get at upolstery shop or a arts and crafts store. Looks perfect now