Congrats on the toy hauler. I have the same truck in a 2019 model and the same B&W hitch. Great choice on the hitch; you should be pleased....
I have my hitch set at one hole below the highest position. Would prefer it to be in highest setting as I would like the extra inch of bed clearance; I tried it there, but it made the trailer ride rough so had to move the hitch back down. The rule of thumb is you want at least 6” of bed clearance, mine is about 5” and I have to watch it close in some situations.
The exact hitch height to achieve the desired clearance is also dependent on where the pin box on the trailer is currently set as there is typically some adjustment there as well.
So I’d also recommend starting out at highest hitch height just to be conservative in case the pin box is currently mounted in its highest position.
Another idea is to ask the seller to measure the distance from the bearing plate on the pin box up to the underside or the “chin” of the trailer. (Recommend you ask him to send a pic of the measurement so there’s no question on what was actually measured.) Based on that measurement, you’ll know how far below the bed rails your hitch can be and still maintain 6”. (I.e if the pin box plate is 9” below the trailer, your hitch should be no more than 3” below your bed rails,) make sense?
Side note which you probably know is make sure you have a good lube plate on the king pin. If the trailer is used and your buying from an individual, I’d suggest picking up a new one and taking with you just in case.
Nonnie & Poppy
2016 Keystone Alpine - 2019 Ford F350 DRW