Still haven't got the picture posting correct.
As suggested in your other post.......white flexible tubing teed into water could be a 'winterizing kit'. Teed into suction side of pump, stick it in a bottle of RV anti-freeze and then pump it thru lines.
Otherwise......if teed into pressure side of plumbing, could be a feed to a faucet, goes to city water connection , ??????
Really hard to tell without a picture OR a good description of where that line tees into plumbing system
ON EDIT:
Here is a link to your other post with a picture and answer (thanks to bshpilot).........
LINKIt is a winterizing kit.
Is it time for your medication or mine?
2007 DODGE 3500 QC SRW 5.9L CTD In-Bed 'quiet gen'
2007 HitchHiker II 32.5 UKTG 2000W Xantex Inverter
US NAVY------USS Decatur DDG31