I have an earlier model of the Pressure Pro TPMS. Works great! I was camping last October up around 5,500 feet and the nighttime temps dipped into low 30's on the morning we were leaving the campground. I hooked everything up and I was alerted to low tire pressure on the trailer. The combination of elevation and temperature dropped the air pressure low enough to trigger an alert. After confirming that's all it was, I drove off and within about 15 minutes I had dropped enough in elevation and warmed up the tires enough to bring the air pressure back up to the correct level.
I also love the ability to check the pressure of any of the 4 trailer tires while on the move. I have now towed over 4,000 miles with this system and it has operated perfectly! You really can't put a price on the safety of your family and those on the round around you, and I believe $400 is money well spent. Pressure Pro gets my vote.
Another TS member (Mustang67408) bought a Pressure Pro after my recommendation and loves it too!
Here's my review post from October 2006:
Doran tire pressure monitoring system.
Since you plan on towing a long distance, here's an experience I don't wish on anyone! In fact it led me to purchase the Doran Pressure Pro system. Read, learn and be safe!
Pay attention to the torque on your lug nuts (long story)!