My MH is very similar to the one you described. Its an Expedition which is the kissing cousin to the Discovery. Same chassis and engine except my tranny is a 5 speed.
The rocker for the engine brake should be on the arm rest console under the drivers side window along with other misc switches. It has a white circle with an R in the middle of it.
The 270 hp engine is adequate for the size you described. I tow an Equinox and yes you will slow down to 35 or 40 mph on long steep grades. But it does give me excellent fuel mileage. I'd quote the numbers but I'd get flames calling me a liar.
The engine is rock solid but at 80,000 miles the external parts are starting to give me maintenance problems. The fan self destructed, fuel pump, belts and hoses etc. But the quiteness of the ride makes it a very good choice.
