I am building some really long skinny cabinets for an RV. (One is like 12 feet long) They will be horizontal kinda like overhead bins on airplanes. The doors will be 12" high and as wide as 48". The doors will open up.

The doors will likely be slab doors due to the dusty areas the RV is used in. What direction should the grain run on the doors? I'm tending to think the grain should run the long direction of the door, but most doors the grain runs vertically. After I typed this up I realized it would be really difficult to build a door like this with vertical grain anyhow.

On a related note, is a slab door 12"x48" going to be prone to warping more than a raised panel or flat panel door of the same size?