I want high idle

OK, I have a 2000 Newmar Dutch star 3883. It has the cummins 8.3 Just purchased it so I'm kinda new to all the buttons. My E-brakes or Jake brakes if you will, I really don't understand. I have a pedal on the left side of the floor that if I push that It will shift the tranny into 4th gear and it will slow it down, as soon as I let go of the pedal it shifts back to it's previous gear. Also on the right side of the steering column there is a toggle switch that someone had put in and it says BC in the up position and just C in the down position and I'm assuming in the neutral position is nothing. I have no clue what that means. And as if that weren't enough on my switch panel under the radio I've figured out most switches but there is one I cannot figure out, it just simply says high,medium,low. Other than that it has nothing indicating what it is, I've tried pushing it as I'm using the jakes but nothing changes.. Don't know... I'll figure it out eventually I'm sure.