I've had mine on the roof for a year, but once the ice and snow are gone I will be moving it to either the rear mask or more likely onto my spare tire carrier. Someplace it is shaded. I consistently get high temperature readings as the sun beats on it.
I attached mine to the Stow-away II hitch mounted storage unit, on the forward side of the unit. I used the 2 inch wide velcro strips that have adhesive backing.
Inside the rear mask is a good idea. I mounted mine on the inside of the basement door nearest where I put the receiver. That compartment is mostly cold storage but it does have the inverter in the front. Our inverter is rarely on, so I’m not getting wacky readings because of it.
I stuck mine on the plastic water tank under the van with a safety cable in case the sticky fails. I figured the mass of water would keep odd temp swings slower
In the utility bay. Sun on that bay in the afternoons makes the reading suspect but later in the evening and especially in the morning I trust it. I do like the idea of inside the rear mask and considered that but couldn't figure out how...