My take is as follows.
A. I have always had a huge issue with SMB's routing of water lines, the flat plate outside the van, etc. from a freezing perspective. That's why I redid my plumbing layout. See:
http://autos.ph.groups.yahoo.com/group/ ... rowse/bc41
where I posted photos of the mod.
B. Since you seem to like freezing temps, I argue you need to reroute your water lines ANYWAY. When I did my mod, I ensured that I could drain my fill line (which is outside the insulation) and still use the interior lines. This means you can't be at an outside water source and pull from it directly to the sink, but whatever.
C. Thus, I'd recommend.... cutting that upper line that goes up into the van to...????? and capping it off for starters. Yes, water will come down through that line when you cut it, but it can't be that much water....? That should solve the immediate problem
D. To fully address the problem, I argue you need to do as I did in my pics: ensure that the fill line that goes to the outside is separate from your distro system so that you can drain it separately. That way it won't ffreeze with water in it again, and possibly crack inside your wall... again.
Note: the PEX tubing they use can take freeze/thaw cycles, but eventually it will fail.