I would guess that your exhaust brake is hanging up and choking off the exhaust. There is an easy way to check. Remove your engine cover in the bedroom, not the one in the closet. The exhaust brake will be right there, but hard to see because of a tin heat shield.
Take a measurement of the exhaust brake arm where it meets the exhaust brake actuator, noting the position. Drive the coach and when it quits acting up, check the position of the actuator shaft and brake arm and see if they're in the same position. If they're not, you have a stuck exhaust brake flapper.
Here's why I think this could be your issue. My 05 Diplomat came from the factory with the exhaust brake not working. My coach would downshift, but the exhaust brake flapper never closed on the exhaust brake. Many hours were spent and a resolution could never be found. I eventually wired a second switch to the exhaust brake so I could activate manually. This basically gave me a two speed exhaust brake. If I turned on the OEM switch, the transmission would downshift. If I wanted more braking, I would turn on the second switch, closing the exhaust brake flapper.
On occasion, after using the exhaust switch that closed the exhaust flapper, I would forget to turn it back off when I came to a stop. The minute I tried to accelerate, the coach would fall on it's face and a huge cloud of black smoke would be produced because the exhaust was essentially plugged/closed. This is what makes me think you might be having a mechanical, sticky flapper issue. Once it warms up, the flapper fully opens.
