Newly rebuilt 727 transmission 1 yr ago. Adding additional tranny cooler so what fluid do I add / use? Tyoe "F" or Dexron III ?

Ford type F was never used by Chrysler, people put it in for firmer shifts ( best when service life isn't an issue)...any Chrysler approved fluid is backwards compatible 7176, ATF+3,ATF+4...or Dexron III (but not if mercon is also listed on bottle )

If you absolutely must put an external cooler on a perfectly engineered system use a Thermal bypass and make sure the fluid goes through the heat exchanger for heating last on it's way back to the transmission 727s didn't like cool fluids ...most Dodge RV should have an gauge or idiot light for transmission temp from the factory.