Newbe question, Mercury Mountaineer max trailer size

Without having actually weighed the tongue, I would calculate 13% of 4500# and add another 80# for the WDH. That comes out to 665# on the receiver. Plus any weight difference of the new receiver to the old.
What did you say the payload of the TV is? And how much weight in passengers and cargo are you planning on carrying inside or on the vehicle?
The payload limitation is the best place to start when estimating what it can tow.