In floor radiant for RV's, yeah on high-end coach bathrooms but sort of impractical for the rest of us unwashed. The most efficient would be radiant using hot water but that energy has to be produced somehow, either with propane (a boiler, water heater, etc.) or very inefficiently using electric to heat the water.
The in-floor radiant is real popular up here in the Midwest for sticks and bricks but there's no free lunch.