Yes, for both external venting and access. Mine is same size x 19.25 deep (to house a 20lb hori$ontal tank). IIRC it is full supported underneath as the sheet metal gauge is fairly light.
I'm reading this now as you're not planning to use the orig propane closet for LPG - that you've got that sorted a different way or don't need LPG. Otherwise, I'd re use it for it's orig purpose.
As for re purposing and building in for outside storage, I'm all for storage, any size. We actually use the orig closet for footwear. But you might be better off starting from scratch, IDK. Depends on what you might want to carry and where to best locate it. If re using the closet, the old door design and latch could stand to be beefed up... I guess the box could be cut shorter too (at the door end), if that worked better for length.
ON EDIT I know (some) people store all sorts of things in the closet, in addition to the LPG bottle, and you do have it already, so might just want to use it... One thing tho, mine acts as a big cold sink, extending deep into the camper. On cold evenings I often imaging the FWC designers having a chuckle over that... So do over I'd prob insulate the outer surfaces of the box.
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