The vent is on the top of the transmission. If its a 99 then it has a 4r100 trans.
The bad of being overfull is if it vents when hot the exhaust can ignite the oil.
If it has a drain on the bottom of the pan you can drain a bit at a time. If not pulling or loosening a cooler line is another way getting some out although sometimes very messy.
I am a Ford trans tech and like to keep these motorhome transmissions closer to the the cold range when hot because of how hot they run and fluid expansion.
We see this so often where I work,the "quick lube" tops it up and it pukes when they hit the mountain passes.
99 Dutch Star Spartan 300 Cummins,Toad 06 Matrix
31 Model A , 2007 Road Glide,
DW Suzie and Georgia the Newfounland