My 02 (I think) Sportsman 400 keeps running batteries dead. Latest one in a week. Ran good for a few days then dead today. Pull starts or jumps off fine. Is this a regulator or something else?

 

I would start with the rectifier(regulator) first as that is usually the issue. make sure to check your connections that they are not all corroded and puts di-electric grease on hem to keep them good.

I also deleted your other post as you don't need 2 for the same issue.