I am not certain that you need the ability to put in a Zip code. NOAA and Environment Canada only use 7 channels in the entire continent. My trailer radio has it built in and it is really easy to scan all 7 channels for the one that you can pick up, it only takes a few seconds. The channels are laid out geographically so you should not get more than one in any location (the closest city). I would say that if you do not have one built into the trailer that runs on DC, get one that is portable and has either battery or crank power (or both) and most importantly one that has an alarm so that when a Tornado, hurricane, blizzard warning etc go out, your radio will sound an alarm and than turn on the broadcast.
NOAA channels:
http://www.nws.noaa.gov/nwr/coverage/station_listing.html
Environment Canada channels:
http://ec.gc.ca/meteo-weather/default.asp?lang=En&n=8830FA44-1
If you are looking for a specific recommendation, Eton has a lot of good models and they consistently rate very well in reviews.
Rob
rvtechwithrvrob.com