I got a chance to work on this project at little more.
Last night, I stopped at the store on the way home, and picked up a 60 foot roll of 3/8" soft copper pipe.
I strapped down one end of the pipe and then wrapped it around the bottom of the pressure tank. I then secured the other end with another ratchet strap.
I have a small fountain pump, which has a 3/8" output on it, and just sucks from the bottom, so it has to be submerged. I put it in a bucket and ran some tube to either end of the copper pipe wrapped around the tank.
I bought some reducing bushings and barb connectors to hook the 4'x10' solar panel up to the pump. The pump has a good flow rate, but low head. I couldn't use it to fill the solar panel. Instead, I had to fill it with a garden hose. I then had to siphon the output end of the system to get the water flowing. Once it did, the pump ran fine.
I also filled the pressure tank halfway up with water. (Couldn't fill it all the way because I didn't have a plug for the side port!) Then I covered the copper with some aluminum bubble-wrap insulation.
After that, I covered the rest of the tank with the expanded foam insulation.
I will try running this system tomorrow if we get some daylight again.
One thing that I have learned so far - it is really hard to wrap copper pipe around the outside of a drum! It's springy and bendy and difficult to get good surface area connecting. It really does need some sort of heat transfer compound.
Once the water was flowing, it was easy to feel with my hand that the copper around the tank was warm, and the pressure tank of water was cold, but who knows how much heat transfer is going through from one to the other!!!