if you are just running a fan and charging phones/tablets a small solar generator power pack thingy could work. You could charge it from the CRV's alternator while driving, plug it in when you are around an outlet, or charge it from a small solar config suitable for vehicles. Alternator-only charging would work best with packs based on lithium cells rather than lead-acid cells since the latter doesn't like incomplete charges.
do a small panel-controller-battery setup, a smaller version of what fulltimers do. Here's an intro into those concepts.
folks with some experience under their belt might replace the starter battery (the one that cranks the engine) with the largest good quality AGM deep cycle that will fit. Either charge just from the alternator as normal, or add in solar for more power during the day. This can work because 1) good quality AGM can pass a ton of current to crank the engine, and 2) a low voltage disconnect (LVD) will keep you from running the "starter" battery too low. If you are the adventurous type you could do this with your existing starter battery but it would have to keep the discharge much shallower to keep from killing it quickly.