Well, you've got a couple of options 1) follow my advice below, or 2) read my laugh-out-loud, thrill ride of a book, Greasy Rider. The first one is probably the easiest. As you know, though other readers here might notdrivers like you and me, who power their cars on french fry grease, tend to stick close to home. I get my waste veggie oil from a dependable local restaurant that sets it aside in a nice sealed container so it doesn't get exposed to the elements or outside contaminants. Then I take it home and use my simple but reliable filtering setup to clean the remaining morsels of chicken tenders and jalapeno poppers floating inside it before pouring it into my car's on-board grease tank. Mmmmm chicken tender morsels.
Venturing across the country means cutting the apron strings of my trustworthy local supplierand scrounging for, pumping, and filtering grease (of dubious quality) from waste oil dumpsters behind restaurants (of equally dubious quality) on the fly. Not exactly an exciting (or particularly clean) proposition. One quick disclaimer on the following advice: it could quite possibly destroy your car's engine, even though it worked for me and my buddy Iggy on our drives across the country. If this happens, please hold the folks at Outside liable, and not me.
Step 1) Buy a portable oil pump that can connect to your car's battery. You'll be using it to suck grease from restaurant dumpsters along the way. Northern Tool and Equipment sells a couple of great ones, including my favorite.
Step 2) Connect a filter system to the pump, so it cleans the oil you're sucking from the dumpster. This is a real trick. Iggy and I originally got this really fancy spin-on filter that would scrub out even the tiniest particles and give the Heisman to any water in the grease. (Engines don't like water, as you know.) But we kept blowing fuses on the pump, because it was working too hard to pull the thick, chunky sludge through the filter. So we went to the hardware store and got a GE Household Water Filtration Unit for about $35. You can buy replacement filters that go inside it at most hardware stores--and the housing has hose connections on it, so you can easily hook it up to the inlet hose so the grease will run through it before reaching the pump.
Step 3) Finding the grease. Peek inside the restaurant's waste oil dumpster first. Most of the time, the grease won't be suitableor there won't be enough of itso you'll have to move on. Once you do find some Grade A stuff, approach the manager or owner about procuring some. (I won't touch the whole strategy of permanently borrowing grease without asking, in case The Man is reading this.)
Step 4) Follow my instructions precisely, here. Say three Hail Marys, then dunk the end of inlet hose half-way deep into the grease Dumpster. Water settles to the bottom, so you don't want to come close to it, and the bigger chunks usually float to the top.
Step 5) Storing the filtered grease in the back of the vehicle. In five gallon, recyclable plastic paint buckets like the orange ones you can get (with lids) at Home Depot. It's easier to pour grease from them than gas cans, especially in cold weather.
I hope that helps. Good luck on your trip. Maybe if your hijinks are laugh-out-loud funny enough, you can turn it into a book that the New York Times Book Review calls "An entertaining combination of On the Road' and An Inconvenient Truth.'" And then shamelessly promote it online. Or has that been done?