I don't know what year your trailer is, but on our 2010 MS 38TKSB3, the switches as described in the answer above is the same as ours. The one switch is outside in the compartment to get to the burner, and one might have to bend down to see it as it might be behind one of the burner lines.
As for the bypass valve, our 2010 has the two-position valve. It is located to the right of the whole house filter as one looks at it from the door on the road side of the coach. See the image below. The valve is the one with the red handle pointing down.
