If you really want something that practically never leaks (unless you overfill it), I recommend the Hellvape Dead Rabbit V3 RDA or, any RDA. An RDA is very, very unlikely to leak, and in my opinion, the flavor is also much better than any RTA you can get. The only things you need to do to keep leaks at bay are, on one hand, not to overfill it, and on the other hand, leave the cotton as dry as possible when storing it in your bag, and that's it. I always carry a mod with an RDA in my pocket and have never had leaking problems; however, with RTAs, it's a completely different story.
Getting closer to your question, which specifically concerns RTAs, I can tell you about the Dead Rabbit V3 RTA, since as far as I know, I'm not aware of any Fat Rabbit V3 RTA (unless by that you mean the Hellvape Fat Rabbit Solo 2 RTA, which is precisely the one in this review). Well, speaking of the Dead Rabbit V3 RTA, I haven't had major leaking problems, although honestly, a key and decisive factor in this aspect is how you wick the coil, how much cotton you put in, how you comb it on the sides...
Another recommendation is to close the airflow inlets when refilling the RTA. That is, you close them completely, fill it, and then open them again; this has prevented leaks for me on occasion.
If you're thinking about buying it/have already bought it, I recommend, if you have an extra device so to speak (any pod device, pod-mod, or any mod with an RDA or a tank with stock coils), that you use the RTA solely and exclusively at home for a week or two to see if you're really sure about putting it in your bag. In the meantime, you can use that extra device and put it in your bag without worrying, if you understand what I mean.
If you have any further questions about this or if anything isn't clear, feel free to reply to this message or open a private chat and I'll be happy to help you.