1. Two external sensors are enough (TM only - my truck has its own TPMS)
That's all I ever bought too. I've used the 507 system by TST Truck for maybe 10 years, and it's been great. See:
https://tsttruck.com/shop/kit-sales-us.html?no_of_wheels=9
2. Its own receiver/display - I really don’t want another app on my cell phone.
Yes, the TST one has its own display. They also sell a black and white display, which will save a few bucks if you can find a kit with one. I don't suspect the color adds any value.
3. Do the wheels need to be rebalanced after the sensors are screwed on?
I have not with mine, though I do have metal valve stems, which I think you'd want for a stem-mounted sensor. You can also consider sensors that mount on the rim, which is how car ones are, but obviously the tire has to be unmounted to install them. They are better, I think, in that they don't need to be removed to check pressure or add air, and I suspect also provide better temperature sensing, especially when the cause of that heat is the bearing. I believe TST sells them.
4. Is a repeater needed? I have a 2720 pulled by an F-150 pickup.
Yes, you will want a repeater. For several years I used the TST sensors without a repeater. It worked great, though occasionally a wheel would drop out for awhile. Then last summer, the wheels would only rarely register with the receiver, but when I took the display/receiver back to the wheels, they would register. I called TST, and they said a repeater was necessary, increasingly so because there is so much more RF interference today, not only from the vehicle itself (some cars' computers connect with cell towers to transmit data), but also owners can have cell phones and bluetooth devices that cause interference. I ordered the $50 repeater, plugged it into a cigarette outlet at the rear gate, and problem solved. Better than before, even.
5. How long do sensor batteries last? Are they replaceable?
Mine last at least a year, sometimes even two years. Three screws open up the sensor to reveal a coin battery, which is easily sourced and replaced.
6. Are the low pressure trip points programmable?
Yes. As are high temperature trip points.
7. I’m not sure that temperature sensing is worthwhile. Thoughts?
I think so, particularly if you're going to go through the effort of getting a pressure sensor, and temperature sensing is also available in the same unit. Ultimately, I think this feature would have more value in rim-mounted sensors, though I like having it on valve-mounted sensors. Even when the sun is shining on one of them, I don't find it significantly influences the temperature when going at highway speeds. Slower than that, and certainly, when stopped, it changes the temp alot.
I find the temperature readings to be pretty consistent, relative to ambient temp -- normally, they are 20-30 degrees warmer. If they read 50 degrees warmer, I'd be pulling over to investigate, especially the hub.
8. Cost - not the first driver, but I see them anywhere from $25-$400.
Sounds about right. The above-linked TST kit, which includes 2 sensors and a repeater, is $333 direct from TST. Not sure how they're priced elsewhere.
I will also say that TST has outstanding customer service. They're in Georgia, and we were having problems last year with our display while on a transcon trip when they suggested the repeater. I ended up getting the repeater from a different source as they could get it to us faster, but when we got it, it didn't work. I called TST, and they said, oops, sorry, that other rep shouldn't have told you the repeater won't work with your older display/receiver, so we'll make it right and send you a new display/receiver for free. Arrived in 2 days. Amazing.
I highly recommend them. Our sensors have traveled 10's of thousands of miles.
Dave
