We have a saying that we tell to all the new guys to MV's:
"If your offered a ride or a drive, take it.
If you want a ride or a drive, ask for it".
This saying has served me well, I've driven alot of vehicles that other people my age don't even know exist.
As for how to get onto a tank, the answer is "carefully" and look for the route that makes sense. Don't jump off either, it freakin' hurts if you land wrong. Also, talk to the owner/driver before you even think about clambering all over it, they know the tricks.