I replaced the caps with valves on the low point drains and leave open when winterizing. No harm Done if you forget to close when water turned on. Happened to me this past weekend. Just reached under and shut off the valves.
2008 wildwood le 19BH
Tv 2018 F150 screw 3.5EB
Retired UNC Charlotte engineering professor.
Mine runs about 12 feet under deck. I turn off valve in basement, and open bib on outer edge of deck before freeze, never had an issue. I don't know if it is sloped enough to drain, as I don't see much water drain out after shutting off inside valve and open outer bib.
As long as there's a shut off inside then no issue . it may be slow to drain or seem like not much comes out because you have the effect of a finger on a straw or low points in pipe between house and bib
One of the few things I use Shark Bite fittings for is frozen and burst PEX run outside under decks to hose bibs. It does burst/split when it freezes. It may not burst every time it freezes, but it does burst quite often.
Of course not a good example to what we are talking about here.
Why would anyone run anything under a deck in freezing weather?
If we needed water and was worrying about freezing the low points I would get a bucket and just turn the drains on ever so little maybe a drop every 25-30 sec. Just that little bit of movement will help to keep it from freezing. Its not a perm solution but will save you a for a couple of cool nights. Watercin your hot water line should be pretty warm even if its ten feet from your tank!