Is the air damp at all? If there's a lot of moisture in the air it can make existing household odors worse and lead to mold and other stuff growing. If it is, you could try some kind of moisture absorber in the worst affected areas, the brand I linked is what I use in my room that gets really damp without a lot of circulation, I usually use a scented kind so it works to get rid of existing odors and prevent new ones if the problem is too much moisture. If it's mainly centered around the baby's room and you think this could be the issue, you might just need a diaper pail that's really good at keeping in odors and/or use a deodorizer in it. In general I find using scented/odor-reducing garbage bags can make a big difference too.
If it's the drain or sink, in my experience just plain old baking soda works pretty well to absorb drain odors if it's not too bad and you let it set in for a while before rinsing and using a general purpose cleaner. I also heard people say good things about stuff like SaniSticks if it is indeed coming from the sink, but I've never really used them. Also, if you happen to have a garbage disposal that could potentially need to be cleaned, but I don't have one so unfortunately can't be much help there, I just know it's a pretty common household problem for people who have them.
If it's kind of even around the whole house, then like you said it could be the AC, maybe something in the ducts or something, or just general odor build-up in the house. You could try one of those scented, air-purifying furnace filters but that might only really mask it and could also be irritating to people sensitive to artificial scents.
Barring all that, as a last resort you could look into a room air purifier, an ozone deodorizer, etc. but usually that's not necessary and ideally you'd want to also find the source of the odor.
You can purchase chemical dehumidifiers at most home improvement places. You want something like this: https://www.amazon.com/DampRid-Moist-ABS-10-5OZ-4PK/dp/B07V3N739V/ref=sr_1_3?dchild=1&keywords=dehumidifier+chemical&qid=1597697846&sr=8-3
I used the Olympian Wave in my trailer here in the PNW...
Between me and my cat exhaling moisture, the plants in my trailer, and the heater, I dealt with A LOT of moisture issues.
I found some relief with DampRid chemical moisture absorbing pots. https://www.amazon.com/DampRid-Fragrance-Refillable-Moisture-Absorber/dp/B07V3N739V
Any type of burning propane produces water vapor as a byproduct. Sucks.
In future projects I plan to use a vented propane heater such as http://dickinsonmarine.com/product/newport-p12000-heater/ or a diesel heater or wood burning.
Good Luck. It's a hard problem to solve...
My bathroom is the same. My advice: shower with the window shut but immediately open it then leave it open until the room dries out - open the window from the top (where the steam goes) and the bottom if you can. Get DampRid or the $1 version of it from Dollar Tree, stash them wherever you can in the bathroom and replace them as soon as they fill up. Get an air circulator fan (and an extension cord if necessary) and aim it into the bathroom so the air will circulate and the room will dry faster.