Once on the motorway, (in UK) I tend to trundle along at an indicated 70mph, which works out about 65mph real speed.
On the continent, I like to keep at about 100kmh (62mph) real speed, on the motorway, as there is more to see that is interesting.
I have seen an indicated 90mph before the noise from the passenger seat made me slow down... perhaps just as well
More often, than not, we bimble along the smaller roads trying not to be a road block, and take in the scenery, pulling over when there are more vehicles behind us than in front.
OK, so my fuel consumption is horrendous at 20mpg, but I did not buy my 4.5t Eura Mobil for its fuel saving capabilities.
I sometimes overtake smaller campervans, but generally they shoot past me (who cares?), I also sometimes get passed by big motorhomes exceeding 70mph, but that is their choice.
I have never tried to do a holiday where have tried to save fuel.