well i drew up a funny shaped tank to fit in the rear side of mine, had it made up in Hull, basically if you can do a drawing it can be made , £60 -90,
as jed just said they freeze under and if the lot freezes then it can take ages to thaw, the last bad winter we had, i forgot to leave a rad on in the van and it froze solid inside too
mm
My Westy Atlantic is winterised...........well insulated to allow all round winter use, but I always drain the internal water tank when parked up at home during winter months.
Daisy our Auto Sleeper has both Water and Waste tanks and I think there great.....apart from they can cause problems getting on a flat bed trailer if you ever transport the camper about.
We have never had freezing problems but then again, Daisy isn't used much in the winter.
Unless you have a water tank inside the van I could see even the barell type freezing outside over night if its cold enough. Having the water inside takes up too much room IMHO.