Hi Patrick
In Europe it will depend how you use your RV.
Do you like to go holiday in one or two places for a period of time or do you like to tour around would you tow a car? Many take 36-38 foot RVs into Europe but they tend to know which sites they are heading for then stay for a period of time using the car to make sorties.
We liked to tour and had a 33-foot RV and car and would cover 20 sites in a tour in France and Spain. I wouldn't have wanted to go any bigger and many sites we went on hardly coped with us and wouldn't have coped with an extra 3 inches. It's not just pitch size it's manouevering in the site and onto pitches. We got into a number of sticky positions (all described in my book).
If you can find sites that you know you can fit on then no problem - just turning up and hoping (like we did) you will have problems and telephoning sites to ask ahead of time doesn't work very well 'no problema' will be ringing in your ears as you desperately try and negotiate their trees, toilet blocks, overhead awning structures, other campers and even their entrance. I haven't even mentioned dumping your waste

However, it gives you lots of stories to tell.
Mo and Dick's Big Pitch Guide could prove invaluable - there is an advert on this site.

Keith