I use a PV trickle charger, the panel is barely 1 square foot or so. Would be nice if it was integrated instead of having to connect/disconnect it constantly. Although, and I'm just guessing, many vehicles that are so seldomly driven are being kept indoors/garaged? (Mine is)
I haven't found any appreciable drain on my EV's primary battery over the longest period I've left it sitting so far (a little over a week, so not that long, admittedly), but the car _does_ do a very bad job of keeping the 12V battery charged and I've already had to replace it once in <2 years of ownership, plus I bought one of those small jump start packs in case it ever dies not at home (luckily, for an EV, it barely requires any power at all to turn everything on and get it started, so the very smallest, cheapest, jump packs are way more than sufficient). A built in trickle charger to combat that would indeed be nice, if the car companies are incapable of figuring out the logic necessary to do it off of the massive primary battery.
EV's are the worst at keeping their 12V batteries charged. Many EV's don't even charge the battery if they're plugged into an AC charger!!!
You can literally leave it plugged in charging for a month and come home to find it dead.