Gary, I had thoought that the Dill #1504 system did NOT monitor more then 4 tires.
That being said,I just spoke with them on Tuesday and very soon they will release a #1506 system with a redesigned lcd screen and softyware that will be able to monitor up to 10 tires (dually + trailer).
I also believe, like you, that the internal sensors are safer then the stem/cap type ones, particularly with higher tire pressures. I do NOT like the idea of a stem type sensor/cap being the only thing that hold the air in the tire. It necesarily works that way in order to sense the tire pressure, it depresses the valve stem 100% of the time, which menas that ONLY the threads the manufactureer supplied o-ring and the cap itself are holding the pressure in. I know that many TPMS systems have done it this way with low failure rates, but to me it is just a bad design idea.
Dill, FYI, has been making valve stems and OEM parts for a long time.
In any case, which model Dill system are you running and will it monitor more then 4 tires??