Ok let's get a few things straight:
- LP gas leak smells like rotten eggs. Will not set off a CO2 alarm
- Fridge leak smells like ammonia (because it is), also will not set off a CO2 alarm
- You can't smell CO2 - it's colorless and odorless
If the CO2 alarm is going off, you have combustion gasses (not LP gas) entering the camper. That's bad. I know you've cleaned your vent top and bottom. The fridge also as a chimney that runs up the back of it from where the flame peep hole is the whole way up. It could be clogged.
Sulfur smell will not set off a CO2 alarm. CO2 will set off a CO2 alarm (again, you can't smell what it's alarming on).