yep, that would be my guess also. You didn't say what year your 5'er is or if the batteries are newer, or how many, or what kind.....kinda difficult to help you out without more info.
There are other things that may be a slight drain besides your CO2 detector. Do you have a radio...if yes, those typically never turn off as they have memory that requires some small amount of power. How about overhead fans, or a storage compartment light...etc.
But it sounds like your battery(s) are just old or maybe a bad cell or something. Since you do a lot of boondocking, it just may be to your advantage to go ahead and get some new ones.
Ron