If you can not find it, build it in!
Tank-Depot.com Look for something that is about 12" tall and 24" deep. The length to place it under either the couch or dinette seat. You can install a second small water pump, it can give twice the water volume while taking a shower, or pump into the other tank if you like. To fill the second tank, you can use a valve to bypass the pump, and then run your other pump, and fill the main fresh water filler. Have an overflow at the top of the tank to go outside, once the tank is full.
Have fun searching and then camping!
One reason most recent RV's do not have a larger fresh water tank is the Cargo capacity rating is reduced with the larger tanks. So a RV with a 50 gallon tank (about 400 pounds of water) will have a 160 pound greater CCC rating if they did not have 20 more gallons in a 70 gallon tank.
So also be careful to find a lighter weight RV, so that you have enough cargo rating to fill the fresh water tanks.
Or you can fit a small tank into your car that you tow along with you. Then fill it while out sightseeing, and bring it back, transfer with a small water pump. You can transfer 50 gallons in 25 minutes using about 5 amp hours of power from the car battery, and then refill the car battery with about 3 minutes of engine run time. That way you do not need to worry about overloading the RV chassis.
Fred.