What would be the most tolerable rpm range for a longer time periods? I mean cruising highways for some (let's say 3-4?) hours... I guess most eco range is about 2000 - would it also the safest for the vehicle with 2,7 diesel engine? Just don't want to make any harm to my RR

Providing it's kept in good condition, properly serviced etc., there's no reason why it shouldn't happily sit at 90mph for a couple of hours - assuming you could ever find a bit of road that allowed a constant speed for 2 hours. You would want to ensure it was given a chance to properly cool down before switching off, however. A few miles at low revs followed by a minute of idling to let the turbo cool down. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed