I just put a Dill TPMS system on my Lance trailer. I decided I liked the idea of the sending units being inside the rim like all the auto manufacturers are doing. It may be unlikely but my valve stems already stuck out far enough and I didn't like the idea of brush or anything else catching on the senders. From what I read Dill is the system used on many semi tractor trailers so figured if they worked well for them it would work for me.
You do need to have the tires taken off the rim to install the senders but I wanted the tires balanced anyway so that was no big deal. Discount Tire mounted the senders and balanced the tires for about $60. I ordered the system online from TireRack.com for $279 delivered to my door.