Probably that wonderful grey PEX pipe. I had a similar problem this spring and had to cut a hole in the bottom of a cabinet in order to access the back of the water heater. I spent less than $2 for the parts but it took me several hours to change it out. Plus I was black and blue from all the contortions I had to go through to reach it.
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Brian, Loretta & Daisy (Golden Retriever)
2008 Holiday Rambler Endeavor 40 PDQ , ISL 400
2014 Ford Explorer toad
All I know is I went to Home Depot and the guy said they did not carry what I was looking for any more but he had a box left in the back and drug it and let me did through it. It was grey and he called it pex. What ever it was it fit and my leak was fixed. When I removed the rest of the old pipe it was so brittle it just fell apart. This was on an 04 National Tropi-Cal
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Brian, Loretta & Daisy (Golden Retriever)
2008 Holiday Rambler Endeavor 40 PDQ , ISL 400
2014 Ford Explorer toad
We had the same thing happen on our way to Alaska in 2007. The only difference was that no dealer or repair shop would touch it. We stopped in every little town along the way to see if the hardware stores had any parts available. By the time we got to Banff we had all the tools and parts necessary to do the job. It took about 4 hours to get the old broken fitting out and replace it with a new brass one.
When we got back I posted the experience on another forum. There were at least 20 people telling me that it would never work and that I was going to ruin the water heater by mating a brass fitting with an aluminum one. It's been working fine now for over 3 years. We went to GNR this year and I stopped by the Atwood booth and guess what, the new Atwood heaters come with a brass elbow mated to the aluminum tank at both the inlet and the outlet.
Actually, brass is ok. What they are really concerned about is steel. I had to use a steel plug for an emergency situation, and three days later it was already corroding. The only danger with brass is cross threading.