Arduino Uno. You're right, I'm using the pin1 of serial TX, could be that a problem?
Do you have pullups on each DHT input? What value?
Yes, there is a 1K resistor on data pin, like in the following schema:
How long are the wires between the mystery Arduino board and the sensors?
That is something that could be problematic, the sensors are placed in a range of 3 to 5 meters with shielded wires.
What is supplying power to your project? Voltage? Current capacity?
The Arduino Uno board is supplied by a Raspberry Pi, the sensors are supplied with 5V. Like in the following schema:
