Matt, my '79 Excella 500 had the booster and M/cylinder for vacuum over hydraulic on the tongue and all the hookups, but was also equipped with drum brakes and the wheels / electrics to match, it took me a while to figure out the brakes were drum / electric. Is it possible yours is set up the same way? everything on mine looked orginal. I wonder if it was optional one way or the other?
'00 34' Classic Limited (LOVSHAC)
'12 3500HD, 4x4 LBZ D/A CC LB