I also like parking up somewhere and using the radio as well with the engine switched off
How much time are we talking about here? A few hours? In that case, operating SSB or CW you're not going to kill a typical car battery. Because the radio will poop out due to low voltage before the battery will get too low to start the car, you're almost assured you'll never be stranded.
But just for the sake of argument, you can determine pretty closely just how many Ah you're going to draw with the particular modes/duty cycles you're going to use and can preemptively run the car to replace used Ah. Even for an aggressive duty cycle at 100W, running the engine for 10 minutes per hour would be plenty.
Yes, you could set yourself up with panels and batteries but buying and managing all that isn't plug and play. You have to store the stuff, schlep it around, set it up, tear it down and monitor it during use not to mention the purchase price plus replacing the battery now and again. Contrast that to just driving to the spot with the car you already have and using the existing battery and alternator which already work. Total extra cost = $0. As added insurance you could put a watt hour meter inline and know exactly how many Ah you've used, and restart as often as you like based on your actual discharge level.
When I go camping I run lights, radios and assorted gizmos and only need to recharge for 30 minutes or so once a day. An afternoon's operating shouldn't be any problem at all.