The title of the thread is a bit confusing, although I think I know what you are asking? Are you asking about the best extinguisher for class A fires? (wood, ordinary combustibles, etc) If that's the question, then the answer is a pressurized water extinguisher. Nothing puts out class A fire better than water with a little bit of foam to break up surface tension and penetrate the fuel.
But I think you're asking about the best extinguisher for a class A motorhome? Nothing special about class A motorhome fires, they are pretty much a house fire and a car fire all rolled into one, with a propane tank for extra fun and games. If I had a fire in an RV (motorhome type doesn't matter), attack the fire with an ABC extinguisher, as up close and personal as you safely can. With any portable extinguisher, getting close to the fire is key. Randomly spraying from a distance is totally ineffective. 2A10BC is a good compromise of cost, size and effectiveness. The bigger the number, the more fire it will put out. After you attack the fire (hopefully someone has called the FD) if you safely can turn off the battery master switch and turn off the propane. If I'm first in on an RV fire, while my guys are stretching a hoseline my first move is to find the propane and shut if off. If all this fails, stand back and call your insurance company. These things really burn.