I wouldn't worry about it, not for 6 hours, even if you left the coil off.
We had a recent discussion in the B190 mailing list on a similar topic. From what I gathered from that discussion, I don't think you have anything to worry about - 6 hours is not a long time by any means. It's when you get into 18 hours or 24 hours that things start getting more interesting.
If it does worry you, you can always buy a bottle of RV antifreeze and dump it in the tanks.
I've camped in my B190 in winter-like conditions - once in Pennsylvania over Thanksgiving, when we actually had some snow (maybe an inch). It actually got pretty cold at night, definitely below freezing, but I didn't do anything special to my tanks and had no problems.
1995 Airstream Classic 30' Excella 1000
2014 Ram 2500 Crew Cab with Cummins 6.7L Diesel
Sold but not forgotten: 1991 Airstream B190
Sold: 2006 F-250 6.0L Powerstroke Supercab