Hi Tim
I have a custom 2010 WDX Double Cab Chassis with a GVM of 3.6T and with the Camper fitted and 285/75/16 tyres get around 16l/100 using my SCAN GAUGE to monitor the engine load, I use cruise all the time and adjust the speed set point to suit the load.
I try to keep the engine load (LOD) below 65% and this works fine , sure it means a bit slower up hills and some gear changes when if I kept the foot down it would go up in 5th but the economy makes me smile, so take my advice ( and many others and buy a scan gauge and smile too!)
It also allows you to check engine fault codes and re set them (they will just come up again it it is real, then look up the code and find out what it means
Hope this helps
Joe
PS I don't have any connection to any svan gauge retailer just trying to help a fellow driver