Most autos (I agree not all, but by far & away most) are checked with it running in park. As far as I am aware Bongo's are the same but I stand to be corrected here if the manual says different. If, as I suspect park is where it should be checked then that is where it must be checked or you can get a false reading, which if it is in D would possibly mean you running with the level slightly high. Not a major isssue but it could result in a little oil burping out if the trans got over excited & a bit hot one sunny day with the caravan on the back. Personally I will not allow my staff to leave a vehicle running in drive or reverse, way too dangerous. I have had guys do it then had the car climb over the end of the ramp (into the wall

) when they (or more likely someone else) gets in the car & touches the throttle. If someone had been in front of the car . . .