I make long trips with my FWC and one off the things I run out of first is water. If there was a way to do it....you should consider a larger thank than 20 gallons
We have an '03 Hawk with a 12 gallon water tank, so this is a problem for us also. Notice the recessed sections in the side walls of our bed in this photo:
I've been wondering if it is possible to get tanks made to fit those. I figure they could be about 15 gallons each and still fit the camper. It would be nice to have one for extra water and one for extra gas. Just not sure if the cost is justifiable.