Propane is a dominant heating fuel in rural parts of the US. Propane users typically purchase a large volume of fuel all at one time. It would not be unusual for them to have to write a check for $1250 1-2 times per year. What this means is that like with a solar water heating system, they are pre-paying for the fuel they use to heat their home and domestic water. Unlike with natgas and electricity customers, it's not great a leap for propane consumers to understand how a solar water heating system would benefit them in the long run since they can't spread payments out over time. Additionally, propane customers tend to come from agricultural communities were communities tend to have a longer view of financial prudence.
To give a little flavor of the economics, I modeled a 2 panel, 70 gallon tank system pitched at 30 degrees facing due south in Des Moines, IA. Using a solar water heater with propane back up tank, the homeowner would save about $450/year in water heating costs (using $2.50 per gallon for propane). Switching to an electric back up tank with the same solar system, would save this homeowner about $500/year. Either system would reduce the home's greenhouse gas emissions by about 1.3 tons of CO2. Even with only the federal tax credit, this system would have a 10-year payback or 6.5% rate of return. If propane prices go up to $3.50, the payback is cut to just over 7 years. If your state has any additional incentives, you'll see paybacks as low as 5 years.
A couple of good resources for propane related information would be Build With Propane and this propane supply outlook paper.