Hi hallremax,
Putting in a larger fresh water tank would involve finding a larger tank that will fit your rig. You'll have to drop the coroplast underbelly and take some measurements. It's also likely you would have to improve the tank support.
If you plan on towing with the larger tank full of water, you should consider that 1 gallon of water weights about 8 lbs. If you were able to fit a 100 gallon tank on your rig, you'd be adding 60 gallons weighing an additional 480 lbs. Fresh tanks are usually located over the axles, so you should consider the effect on the axles, suspension, wheels and tires.
If your truck can handle the additional weight, an alternative might be to carry an auxiliary water tank in the truck bed and use a 12V DC pump to transfer the water.
Here's one.
And of course you need to consider where the used water goes. Are your gray/black tanks large enough to handle the extra water?