Almost all major network and cable sites have copy and download protection on their videos. There are lots of browser download apps around, such as DownloadHelper for Firefox, But even when they download something, copy protection prevents saving the file permanently or replaying it. Now and then some Ukranian hacks program a downloader that works briefly, maybe a day, a week, a month, but the sites continually change their copy protection codes and often store those videos as numerous segments. There are several years of posts at Videohelp by people who suggest various ways to bypass unknown protection schemes, but either they don't work or they're only temporary.
Have you considered an iphone or similar device that connects to the internet anywhere? Add a car recharger that fits in the cigarette lighter and you're good to go all day. Otherwise you can find Google links with dozens of solutions, but you take your chances that they work or not.
Several newer gimmicks are listed at videohelp.com, but user reviews vary. One is
VDownloader, another is
ChrisPC Free VideoTube Downloader (the latter is free but earns its keep by downloading a ton of junkware with the installer). You can try videohelp's software
Tools page. I gave up on all that stuff and continue to carry my trusty iphone on driving trips.