There should be two outlets on that tank. One out to the pump and the rest of the trailer. The other is the drain to the outside, usually with a small plastic spigot visible outside the unit.
The plastic tubing between the tank and the pump and the tank and the outside spigot can deteriorate over time. It is also prone to breaking if frozen. Those pieces of tubing should be checked before trying to remove the tank. They are fairly easy to replace and the tubing and small worm clamps (if needed) should be available at any hardware store or home improvement center.
Next, there are nylon fittings screwed into the tank for these two outlets. They should have a "barb" on one end for the tubing to fit on, and a threaded section on the the other. These can also break, but probably not due to freezing. More likely they'd break if they were struck by an object. These are also common plumbing parts available at hardware and home improvement stores.
If the leak is in the tank itself, there are patch kits available that will essentially "weld" the crack or puncture.
Looking at the floor plan for the '88 T2260 series, it looks to me like the fresh water tank is most likely under the front couch which is on the front wall of the trailer. If it's there, a couple of screws should remove whatever wooden panel is under the seat cushions exposing the entire tank.
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