i highly suggest not to ask, just take. its kind of a philosophy of life. but realy, i've put on 170k miles on grease back and forth around the country and it pays to take it when you find good grease and not ask. if the say no then they watch you leave, if you dont ask and they catch you, then youve already got some fuel and you peace out if they've got a problem, well before the pigs even get the call.
as for places, i dont want to give my places away over the internet, but there are some national chain resterants, that always have loads of clean oil, and are on the side of the interstate in every mid size town. just find the right ones and keep looking for them in the next town.
unfortunatly you have a very small tank and have to fill up often, but i suggest getting a roof rack for hauling extra cubes of grease, since the mercedes's want to sag alot if you pack the truck with oil. definatly dont put more that 16 gal in the truck (including the grease tank) or youll be popin wheelies down the road.
personally, i have a 100 gal tank in the back of my dodge and i can drive from n.c. to denver without stopping if i want to. i can feel the weight a little, but its centered between the wheels in a truck, that's why i suggest putting reserve grease on the roof.