Larger fresh water tank ?

JMO, but this is far more work than I'd ever want to put into a mod like this, especially considering that no matter how large the FW tank is it'll never be "enough" and chances are you'll have to have an efficient method of refilling it anyway. That's what I'd put my effort into - fresh water transfer i.e. a means to collect more fresh water as you need it without involving the trailer at all and a means to efficiently transfer that water into the trailer's FW holding tank, whatever size it may be. That's the approach I've used and now essentially have an "endless" supply of fresh water. :B