Not all cars have TPMS, or even if it is fitted not all give pressure readings
You are probably right that not all cars or caravans have TMPS, but I was saying what I have and how I exploit having it, no more or less.
In respect to valves leaking, disturbing by unseating a Schrader valve has to be a primary cause for an otherwise non leaking valve to initiate it leaking.
That the chances of a Schrader valve leak might be very low, does not alter that disturbing it is way more likely to initiate a leak that simply leaving it undisturbed.