I just came across these on eBay. I have NO idea if they are any good or not, so take it for what it's worth. For $20, I figured that I couldn't go too far wrong.
These do not read out a constant tire pressure. They only warn you when the pressure gets low. I have no real desire to "Monitor" my tire pressure anyway. I just want to know if I have a sudden pressure loss on the trailer. You could drive quite a long distance without even knowing that you had a blow-out. This should warn me if I have low pressure or a blow-out and that's all I want.
I'll let you know how they work when I get them.
http://cgi.ebay.com/ebaymotors/SECUT...item563ac1b47a