6 or 8" PCV pipe. Get you self a length of the largest PVC pipe your local DIY store has, get 2x T's / 2x End caps, few adapters + garden hose and your done. $30.
Not the best pic, but you get the idea, the black tube on the roof rack.
Just pipe with a T at each end, one end the T points up and had a fill cap, and other end the T points down and has a garden hose quick disconnect. Both the T's are capped.
Holds about 6gals, freezing is not an issue, the pipe is way to thick, I painted it black so it heats the water up during the day. In the fill end I also added a schrader valve so I could pressurize it if needed, but I never do that unless taking a shower.
I used a T at each end rather than an elbow fitting since I couldn't find an elbow that went from 8" to 2", and I found T fittings that had 8",8",2" outlets. Obviously for the fill and hose connections you need to reduce the pipe size.
I just keep the hose coiled up inside the back of the van.