I took a close look just now and the hydraulic controls on the sawmill are 20" from the hitch ball. The camper extends about 20" past the hitch ball also. The mill, as it sits now is just about 30" tall at the controls and the camper is just over that. So, considering that, an 18" extension would put the controls well past the end of the camper and unless I got into some tough terrain where the camper is going up a hill and the mill is going down one (saw crossing a low spot to get to a mill site) I think the 18" extension may actually work.
For the cost it's worth giving it a try so I'll order one and see what happens.
I'll let you know!