You don't really need anything as large as a car battery cable (4 AWG) for an Airstream. 8 AWG is capable of handling 55 amps, which is more tha you will ever draw in your trailer on DC. My '06 CCD also has 8 guage cabling from the batteries. Your car needs the larger capacity cabling to provide a large burst of amperage for your starter motor.
No worries! Just be sure that the cable is in good condition and that no corrosion is present upstream from the end fittings. If there is corrosion present, you might want to consider replacing the cables with new ones that have heat shrink seals on the ends.
Also, be sure that all of your battery connections are clean and tight as loose or corroded connections will create loss of voltage flow and will create heat at the terminals and possible arcing.
Lew Farber...ABYC Certified Master Marine Electrician...RVIA Certified Master Tech ...AM Solar Authorized Installation Center...AIRSTREAM Solar & Electrical Specialist