I cannot think of a reason for the added tubing. It looks like Airstream's 1967
era flare work, but the polyethylene tank would have been specified to have 1/2" nipples and hose
what looks like threaded and glued attachments. If the tank doesn't have a flange or nipple molded into it to accept the copper, it's got to be an Airstream afterthought or the work of a Previous Owner. If it were clear tubing, and not copper, I'd guess it could have been a sight to indicate water level. It's not illustrated in the factory plumbing diagram.
My guess is that someone thought that it could somehow act as a vent, but your fill cap is your vent, so the tubing seems like a misunderstanding.
Maybe someone knows why it's there, I'd say it's unnecessary.
Funny you ask about the floor… I'm almost finished with a two page-long rant about floating loose-lay floors VS engineered glue-downs.
Engineered T&G maple 3/8” x3”, Pre-finished with titanium oxide poly, lightly eased edge, glued and nailed is my choice as “The Best”.
If you're interested in further support for that statement, it's thorough and convincing.