I've seen some pretty nice DIY cabinet installs. If you're DIY challenged a cabinet shop would probably be your best bet.
Storage is always in short supply with the 139's. Our FK model had the fresh water tank in the rear curbside under the bed. Since we never boondock (at least we haven't yet) we removed the tank and added an outside access door for more storage. I can always put the tank back in later. Towing with a full size van really cures a lot of storage headaches.
That area is where most 139's have the entry door. Our entry is to the front.