I'm not an expert on solenoids but use several on the boat (though none for gas) and none of the ones I've felt are uncomfortably warm.  Given propane's flammable and explosive properties I can't imagine that's normal or especially good.

Ben

Hi Warren, My boat has a fairly common 12v Fireboy Xintex S-2A propane controller, and today I used a laser temp measuring pistol to check out its solenoid. What I saw surprised me a bit but is probably just what happens when electrons activate a good magnetic flow switch over 20 minutes or more. The temp difference between the Maine air and surrounding propane cabinet was about 30 degrees -- the solenoid about 90, the rest about 60.

I don't know for sure but I suspect that's how it's been running for maybe 20 years, 10 under my ownership. And the boat has not blown up. But if your solenoid has a greater temp delta than that, or if it's skin-burning hot, I'd be concerned.