Can I release without a specific city
Yes, you can. There are special release zones for US states and for the US as a whole, for "-- Highways, roadsides, and services", which should cover the kinds of things you're talking about. Or, if you know specific rest stops/eateries/etc. that you're likely to hit, you could check the appropriate state/town to see if there are existing zones for those; I do like to add little red numbers to as many states and towns as possible when I'm on a road trip, even if it does mean doing some fiddling to figure out which town a particular interstate rest stop is in!
Or, you could use the journal-entry method, as you mentioned, with the option to add more accurate release notes after the fact for books that haven't been caught yet.
FWIW, I generally use a mix of release-notes-in-advance for spots I know I'm going to hit, and release-notes-after-the-fact for everything else. On the rare occasions when I get a catch before I've made release notes, I edit the previous JE to add the release info and any pictures.