Any excuse to fundraiseI would get it uprated. You won't have any significant problems in France. We are 3.8 tonnes and have never found this to limit our access, other than in places where yoou would not want to take a motorhome anyway.
General thought - we regularly discuss issues of motorhome GVW and up-rating on this forum. It is an open site so do you not think that enforcement agencies might look from time to time and see comments like 'most motorhomes are overweight'? That could be enough to prompt a more thorough approach to roadside testing!