Your profile is incomplete - it should include what your TT and TV are - make, model, year, etc. This will help people give you more appropriate answers. If you're towing with a pick-up truck vs a small SUV, they would give two different types of answers.
I'm guessing you don't have a truck and room might be tight. I'd by a couple of these collapsible water jugs. After you use the water, collapse them and they won't take up so much room. And with the valve, they're easy to get water out of - you don't have to lift them each time.
5-gal collapsible water jugBill