esimmers' very interesting post
"Inside Cabinet Take-Apart" from 2010 may be helpful to you, hoyden. It's of a 2009 Hawk and that's a different water pump than yours but at least it has the hot-water bypass piping that FWC started installing in their campers in 2008 or so. And it has the optional water-heater and shower connections. Also, I wouldn't be surprised to learn your system doesn't have the coil of clear hose shown in the diagram but rather a pipe going back to the fresh-water tank.
I believe that diagram at the bottom of post 2 is the only schematic I've seen of an FWC water system.
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I'm not sure what you're asking in your question about a pipe behind the water heater tank. If you're asking if there's a pipe behind that foam insulation we see, then, no, there's no pipe there. That's the insulation covering the back of the aluminum water tank.
Photos of an Atwood 6-gallon DSI water heater on the Panther RV Products site. (And of course if I've misunderstood the question, please clarify)
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I imagine you suspect you have freeze damage because of the water or ice you found on the floor and you're trying to figure out where the break is.
Freeze damage is often a split pipe or a fitting pushed off by the expansion as water turns to ice. Water could also have come from a split in a water-pump or strainer housing and of course a split in the water-heater's aluminum tank.
I see you say you've not used the water heater. I wonder, though, if the tank was filled, perhaps inadvertantly. That of course doesn't matter if your winterization process drained the tank. What I'm getting at is you may want to think back through your winterization this year for a clue as to what component may have frozen.
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