I have an RHP. I built my own thermostat (EECS major ) and I use a combined humidity/temp sensor for it that sits high on the hot end of the tank.

For me, this was so that the humidity sensor was far enough from the humidity output of the humidifier (humid...) and then I played with the tolerance on my thermostat until the ambient heat was where I wanted it.

This seems to be the important part, IMHO - ensuring it is adjusted to where you want it after placement, more so than worrying about exactly where to put it, especially with RHP's, which I think do a good job of evenly heating airspace, as compared to something like a UTH.

Jim