Having just upgraded to a tag-axle rig, I happily ordered a set of extra sensors from Daryl and Cheri at Lawrence Electronics Sales.
They sell TireTraker TPMS for RVs. I've had great luck with the product; and, when I called to order a new sensor after a catastrophic dolly tire failure launched one into orbit (it faithfully transmitted the loss back to the receiver as it headed off on its one-way doomed flight), Daryl wouldn't hear of me paying. I protested, saying there was no way he in any way was responsible for an arbitrary tire failure, but he held firm (for the tires' manufacturer, Carlisle, it was a different matter).
The icing on the cake: after submitting today's order, I held my breath as shipping and handling was calculated. Such is the fashion nowadays, with vendors gouging the dickens out of customers after they've already committed...
Are you ready for this? USPS standard delivery, packed and shipped, for $2.95.
Now, some two years after my initial purchase, I like my TireTraker system; but I LOVE Lawrence Electronics Sales.
Kudos to Daryl and Cheri!
Followup: I got a personal note from Daryl, in addition to my emailed receipt. It is SUNDAY. They have a customer for life.