Three things in that payload, I tripped on two of them with forklift trackers.
The map card’s Longitude column defaults to lng, Latitude to lat. You send lon, so every row has no longitude and gets skipped. Either rename the key on the device or set Longitude column to lon in the card. I’d rename on the device so every future board just works.
You’re sending the numbers as strings because you built the JSON with String(). Number-like text often works in comparisons, but send real numbers anyway. And String(float) gives two decimals by default, which is about a kilometre. Your trailers would all sit in the same spot.
And don’t publish when the fix isn’t valid or you’ll get trailers parked at 0,0 off Africa every time one boots in a shed.
if (gps.location.isValid() && gps.location.age() < 3000) {
char buf[96];
snprintf(buf, sizeof(buf), "{"lat":%.6f,"lng":%.6f,"spd_kmh":%.1f}",
gps.location.lat(), gps.location.lng(), gps.speed.kmph());
client.publish("v1/telemetry", buf);
}snprintf with %f works on the ESP8266 core, unlike on an Uno. Card settings are on Map cards.