I thought about a repeater but placed the suction cup holder on to the inside of the back window of my truck. It is where I can see it in my rear view mirror if the alarm goes off. It is close enough to get the signals and although I can't touch it, I don't need to. Works for me.
TST 507 is a very popular TPMS and will handle most trailers (5ers and TTs) with no problem. And TST has a repeater for those that do need extra signal. Mine works fine on my 35' 5er.....most of the time. On rare occasion I may loose communication for a short time. So I did buy a repeater for more reliable signal.