Are Mobilis tickets valid to those airports by local bus to another station? That might be a sneaky way of getting to the airport without paying the airport premium.
They are, but local buses aren't that numerous in the areas of the airports:
For Orly
Valid on Buses 183 and 285
183 Porte de Choisy Metro (Line 7) - Orly South Terminal
every 30-40 minutes, no late evening service (last journeys around 2030/2130 dependent upon direction). Check the destination, as there are short journeys on this route, particularly on Sundays.
The journey takes an hour from Porte de Choisy, and adding on the 15 minute metro journey from Châtelet means I cannot recommend this route.
285 Villejuif-Louis Aragon Metro (Line 7) - Orly South Terminal - Juvisy RER (Lines C & D)
every 7 - 15 minutes daytimes, every 20 - 40 minutes evenings and Sundays. Service runs 0500 to 0100 approx. Journey time 20 minutes from Villejuif-Louis Aragon. Villejuif-Louis Aragon is 20 minutes from Châtelet by Metro 7. Alternatively, journey time 15 minutes from Juvisy. Juvisy is about 20 minutes from Notre Dame or Châtelet depending on route, by RER. This makes the total journey time from central Paris a little under 1 hour.
For Charles de Gaulle
Valid on Buses 350 and 351
350 Gare de L'Est - Gare du Nord - CdG Terminals - Roissypole (one way loop in Airport)
every 15 - 20 minutes daytimes, every 25 - 30 minutes evenings and Sundays. No late evening service (first journey around 0530, last full-route journeys around 2130)
Note that this route takes a one-way loop around the airport, and there are variations between the daytime and rush-hour route. Journey Time around 1hr.
351 Nation (Metro & RER) - Galliéni (Metro Line 3) - CdG Terminals - Roissypole (one way loop in Airport)
every 25 - 35 minutes, 7 days. No late evening service (first journey around 0530 from Nation, 0700 from CdG; last journeys 2020 from Nation, 2130 from CdG)
This takes a different one-way loop around the airport, and there are rush hour variations as well. During rush hours, short journeys operate on the southern section, so check your destination at Nation. Journey time over 1hr from Nation, 15-20 minutes less from Galliéni. It's about a 15-20 minute journey from central Paris to Galliéni on Metro 3. Nation is a major Metro interchange in eastern Paris.
As these are day routes, there is no overnight service on them, but night service is available on Noctilien routes N140 and N143 to CdG (N143 is the faster and more frequent service), and N31 and N131 to Orly (N131 is substantially faster)