Just a long shot. I installed a Force 10 6 gallon in 2000. About 6-7 years later, it started leaking coolant around a hose connection. The threaded nipple on the water heater had corroded inside a threaded couping, that connected to a barb fitting. I removed the coupling and barb and ran direct, over the threaded nipple. It's still fine after several seasons since(must have been a dissimilar metal problem) .

If the outside is rusted, you're probably wise to replace yours, no matter what the problem is.