I use the ice chest method, with car. As an added twist, I no longer use the big 6-gal hard plastic cans--mostly. Rather, I use Nalgene 3L soft-sided water bladders like this:
http://www.rei.com/product/626195/nalgene-wide-mouth-cantene-96-fl-oz
Yes, they're expensive, but they'll pack well into a cooler. Also on Amazon there is a 10L Chinee Cheepee version that hasn't leaked yet and is almost free.
In the morning when the sun hits the windshield I'll lay some out on the dash and warm them up. If you have a microwave you can stick them in and get them started. Unscrew the cap and lay them in a shallow tupperware.
With these smaller soft-sided water bladders it's easier to find small spots in your TT to store them. This is the best method, of course.
I've stored jerry cans in the shower, as others have suggested. I've also attached holder cups in the shower for a clothes hanger bar like you see in closets. I remove the bar when I take a shower and the rest of the time I have a lot more storage room available. Not only can I hang wet clothes to dry, but I can hang duffel bags full of random junk that would otherwise be in the way.
