1,000 gal tank halfway is plenty of water for it. What kind of filtration do you have for the tank? turtles in general are probably the dirtiest things you can possibly keep in a tank... they poop a lot and stink up the water very quickly. I'd say you're golden for a few years but once it gets past 1' long the waste may overload whatever filtration you have for the tank.
Oh if you definitely need to keep it in a car for that long, I'd suggest get it a big plastic water container, ~40-50 gallons and fill it up 3/4 with cool water, make a platform and close it it partially. the water will not get too hot even in full sun.
hope to see some more pictures soon