When we did the install on Vintage thunder that was the measurement we used. The distance to the saddles can be "tuned" using the threaded turnbuckles. If Reese says 18 inches in the printed instructions I would go with that. They have a reason for recommending that as the correct distance based on engineering. If there were newer instructions I would assume that they would be on the Reese site
Link to PDF of the instructions
The 18 inches is to the top pivot point on the sway control. The installation orientation of the bracket makes no difference, it is purely a matter of having mounting options. Please review the instructions, and I bet you will find that the other installer was not measuring to the pivot point. I had to read the instructions 3 times before I drilled holes on VT.