Sorry. Don't know about installing "larger" size fresh water tank. Don't know if it will change the weight balance of your TT (remember the golden rule to keep 13% - 15% of loaded trailer's weight on its tonque), if its frame will hold the extra weight, or even what size of tank straps one will need.s Lots of engineering questions to answer - before implementing a large size tank.
As a temp solution, perhaps one might want to buy a fresh water dispensor. And, take along 2-3 water tanks. For example: http://www.electrotemp.com/Product-C...agma-DeepClean
With above portable unit, simply transport the system and its bottles in your TT's front and rear storage compartments (to keep trailer's weight balance). When you get to your CG site, simply "set it up" - like your awning and other items. Being a skinny shape device, one could set it up inside your TT or outside. And with this system, one can have refridgerated cold, hot or normal temp water - if plugged into 110/120 AC outlet. Great for CG camping sites - that have electrical service.
Hope this "low cost" temp solution helps - until your long term solution is installed.