Lots of ideas/opinions/theories. The ATF circuit is a closed loop (unless it's leaking ). There's lots of concern/interest in the many (oft posted) bar graphs of ATF temperature vs ATF life. Of course the graphs don't specify where the temperatures are measured but if we ASSUME it's the maximum temperature reached in the system, then IM(H)O that would best be measured where the oil leaves the transmission.
https://www.google.ca/search?q=atf+life+temperature&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiS3N2LmZ_JAhVHHR4KHd39DWMQ_AUIBygB&biw=1252&bih=581#imgrc=cHBKVoRY2PVniM%3A
That said, I don't know how much the temperature will differ between the various possible locations because I don't have a fully instrumented MH to check them. Also, if I had an ATF temperature gauge installed, I'd more than likely use it to watch the changes in temperatures under similar driving conditions to monitor for problems. If it (for example) usually showed 180F cruising on flat ground, rising to 210F on steep climbs, so be it. But if suddenly it was 210F & 230F, I'd investigate. The same way I'd use the coolant temperature gauge.
But that's just me.