When I read your description it at first sounds like a trailer with too much tail weight. But you are flat towing so that shouldn't be a problem.
However if your coach is extremely tail heavy it might be exaggerated when trying to pull. Have you noticed if the handling feel changes depending on if you are accelerating or coasting? How about the tow bar, is it fairly level, nose up or nose down?
How does the MH handle when not towing? would you put handling back up to an A or a B? I would try an alignment shop as there might be a worn part or parts that is contributing to the wandering feel.