Steve - Our bus came with Jake's installed. Before our bus I have had zero experience with driving any heavy equipment.
Our bus has a 6v-92T with a 740 auto trans.
When at the high end of 4th gear...65-70 the Jake's don't make much difference. They help but not a whole lot. If I get down to the lower end of 4th gear 55-60 the Jake's can hold that speed on a 5% grade. If its a little steeper then you have to occasionally apply brakes to keep the speed.
I love them. They add additional braking power and there have been many hills that would have toasted my brakes (mostly because of my lack of experience) if I had not had the Jake's. They also cool the engine. On hot days at the top of a long climb I've seen my temp gauge go up to 195 and as soon as I crest and apply the Jake's it quickly drops back down to 180. Without applying the Jake's the temps seem to hover on the high end and are slower to come back down.
If I had a bus without them....especially a manual transmission it would be hard pill to swallow to spend what it costs to buy, install and correctly configure the brakes. Once you had them in though...I don't think you'd regret it but I do think you'd always be trying to assure yourself you made the right choice in spending the money.
Lastly - your bus sounds bada$$ with the Jake's on. Nothing more fun for the kids than hitting the Jake's when you are cruising through a tunnel.
Fulltiming somewhere in the USA
1984 Eagle 10Swww.herdofturtles.org