Sounds nice. But a word of caution, when you add to the tongue you are adding tongue weight and in the case of a water tank potentially a substantial amount, depending on the size of the tank. So if you are talking 5 gallon tank, no big deal. A 55 gallon tank and you might be overloading the tongue weight of your trailer. Once again, depending on the size of the trailer and frame. You didn't say what size trailer? Single or tandem?
55 gallons of water weighs roughly 458 lbs. We take two drums of water in to our camp is where my knowledge comes from.

We use a 16 foot box/cargo trailer to transport/store some of our camp as well. Things that are only used a couple times of year (i.e tents/frames/stoves). Really nice to travel 800 miles and know that your gear is safe and dry upon arrival.