Mine is in the front battery compartment on the wall. I wired it in line with a switch so I can turn it on and off. Like others said, I get pressure reading fine when it is off, but on occasion have noticed they do not refresh or may drop. I have no issues when it is on.
My new 3791RD just got to the dealer yesterday and I will be needing to install mine as soon as I take possession of the trailer. I will look to put it in what would be the battery compartment, but since I got the super solarflex system, there are no batteries there. I'm hoping there is still some power available.
I tie-wrapped mine under the rear bumper and crimped it into the tail lights. Took me longer to look up the tail light wire color code on the web than to do the work. Now it only works when the lights are on, if I forget to turn them on in daytime (auto on at night) the trailer tires wont register in the display telling me I forgot to turn on the lights.