I think either one would work just fine.  The only concern with the U bend might be water trapped in the fill line somehow causing problems.  I don't think that would actually prove troublesome, as that hose should not be harmed by having water frozen in it (the fittings on the ends are a different matter, but the hose looks plenty flexible).
The 2x4 would not be weakened enough to cause any worry by drilling through its midpoint.  The strain on the beam towards the middle is lower than near an edge, of course.  Most if not all commercially produced RVs us much less stout framing and they hold up well enough, so you also have a lot of margin to work with.