I know that was a surprise!
Here's one place that has the tanks.
RV Water Tanks
There are several places that sell the tanks. Just search for RV water tanks and you'll find plenty.
You'll need to measure the area that the tank fits in and get one that will fit.
Your unit had 60 gallons of fresh water capacity, so it probably had a 52 or 53 gallon tank. (Yeah they count the water heater tank and everything else as fresh water capacity.)