Alan,
you do not need an app. I have used one of these with my Avtex TV for a few years now and it is brilliant.Before we go away I just do a quick internet search to find the local transmitter and make a note what direction it is from the campsite. If I forget I look around to see where any houses have there aerials pointing.
Sucker cup it to a window you do not plan on opening but make sure you clean the area first. Mine usually falls off first time and then is fine.Aim where you think it should be, tune the TV and wait. Sometimes you might need a bit of fine adjustment so choose a weak/braking up channel and twiddle the direction slightly whilst listening to your partner shout "helpful" comments from within..

Colin

I believe my Avtex aerial is the STH3000. This comes with a small in-line unit that you put on the aerial cable. This has a 12v cigarette type power feed jack lead that you would need to get power to the aerial.
I bought a 2nd hand 16" with built in dvd and satellite tuner about 3 years ago for £150 as new they are expensive. You could try contacting Avtex directly as they have returns/ex-demos for sale. I bought a 10" version like this from them prior to the model I now have.

Colin