TST 507 for me also! I was having an issue with the monitor losing signal after being on the road for about an hour or so, so I called TST. They told me that my the monitor was probably going bad and sent me a new one at no charge. On the next trip I had the same problem so I called TST again and they said that my trailer length put me right on the edge of needing a repeater. I ordered on and installed it on my TT. I haven't taken a trip with it yet, but TST gets a positive review from me for helping me solve the problem. They even called me to see if my problem was fixed! In my opinion, that's pretty good customer service!