Madjack & Mike,
I never thought about using plywod, might have to think about that. I have some nice birch doors that I picked up a while back that I would like to use for the kitchen, so I was thinking about using birch for the framing. For the interior cabinet, I thought about making sliding doors out of some of the scrap birch 1/4" plywood that was left over from the sides, and again making birch framing. My main concern is how to hold the birch framing together if I go that route.
I dug through my sons stuff, and found his dowel jig (he left most of his woodworking tools with me when he was stationed overseas

) I also have a biscut jointer attachment for my router, so those would be the easiest/cheapest ways for me to fasten the frames.
I've also been wondering about attaching the cabinet frames to the side walls. I have sandwich type walls, and put blocking in where the cabinets will attach. Was thinking about screwing them in from the outside of the trailer (before putting the aluminum on), instead of from the inside, so none of the screws show, and because I thinki it would be stronger, but wonder if I might regret that if I ever need to remove or repair them later.