Is your vehicle a pickup truck? If so, probably the least expensive way to power the freezer for a quick trip would be a little gasoline generator. The 900W harbor freight two stroke would probably do the trick just fine. It won't be especially quiet, of course, but that's of little concern on the highway. There are, of course, several much better small generators for more money. If you do use a generator, don't start it under load with the freezer plugged in.
(I still believe it's entirely an unnecessary complication to have a running freezer for this short of a trip.)
