I installed a Gera Vendors on my 87 Suburban. I was towing a 34 ft Airstream. While the GV was a great improvement in the limitations of the stock transmission I would not say there was any improvement in gas mileage that could ever be realized in the life of the truck.
$2,000+ is a lot to put into that older rig.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles