When that happened to my Winnebago Class C, it was the famous vacuum leak. Squirrel chewed vacuum hoses, to be precise. When you looked down into the engine, you could see a squirrel nest. I had to have all of the vacuum lines under the hood replaced.
I then took a 12v tail light bulb and socket and put battery connections on it. I placed it under the hood so the engine compartment was always lit up when parked at home, and left the hood cracked open so the critters would not feel secure in there. I kept the MH plugged in 24/7/365 with a Trik-L-Start sending power to the engine battery, which kept the engine battery from running down from the constant light bulb.