THere is no Best you can replace it with just about anything you find at Lowes, Home-Depot, Wal*mart your local hardware, Or any other store.
THERE IS NOTHING SPECIAL ABOUT AN RV CO DETECTOR.. They are identical to units sold for Sticks and Bricks
Right now Lowes has a special on FIrst Alert with the 10 year battery THis is a dual (Smoke and CO) detector, and it is one dollar more than the replacable battery version of the same alarm.
You may need to replace the mountaing bracket and the screw holes may not line up but odds are the new bracket will cover the old holes.