Don, I'm betting your toad is farther away from your dash sensor than the axles on my tri axle trailer. But still, my truck will pickup the trailer tires when it is parked 150' away on the other side of my building. I had bought the booster with the system based on their recommendations of distance, but I never installed it, no need. And I am just using the little antenna on the unit, not the big wired antenna that also came with it. Apparently there is some variation.
I have the older version (that they still sell, a little cheaper) where you can't replace the batteries, you have to send the sensors in for upgrade when they quit, seem to get about 4 years. The new version has replaceable batteries you can change out your self. I'd suggest that version. Also, buy a spare sensor. I've gotten so used to the peace of mind, that I like having the spare sensor on hand, that way if something happens to one I can keep rolling without worrying about that tire.
I bought mine from Camping World, but they dropped TST for another brand a couple of years ago. You can buy direct from the manufacturer:
http://www.tsttruck.com/Home_Page.html
$300 gets you the first 6 sensors and the dash unit, they sell larger kits, figure $50 per sensor added to the kit price after the first 6. Sounds a little pricey until you think about the cost of one or two saved tires, or the damage to the vehicle if a tire blows bad. Even worse if a tire blows on the toad and you don't notice real quick, there are horror stories out there of the toad burning to the ground from a tire fire, or crashing. I like my peace of mind and recommend this or some similar system to anyone who will listen.
One helpful hint, you do need to have the solid high pressure bolt in variety valve stems in your wheels (which big rigs should have anyway). Passenger car tires for your toad, or a lighter trailer may have the rubber type valve stems. If so you will have to have the bolt in stems installed. Make sure you tighten the nuts on the valve stems way tighter than you normally would, so that the valve stem won't turn in the wheel when you are installing/removing the sensors. And tell them you want an extra removal tool or two, they will send them along at no extra charge if you ask. The only way you can get the sensors off the wheel to add air is with a tool, so you are screwed if you lose it. Ask me how I know this. I keep one in the door pocket, a spare tool in the glove box, and one at the shop. I would keep one in the toad as well in case you need to add air when away from the truck.