We’ve said it in print. We’ve said it to Mercedes-Benz. We’ve told our friends. We’ve even admitted it to our therapist: The Mercedes-Benz R-Class has a weight problem. Tipping the scales (tipping them over, actually) at around two and a half tons, the R has no natural enemies in the automotive arena — it looks like a Sumo station wagon, but any BMW or Audi luxury wagon you’d try to put it up against would get lost in its massive shadow. And while the all-wheel-drive R-Class is built …