They added certifications last week (I think) that can increase the idle timer to up to 30 minutes. They're under "passive system".
EDIT: I just noticed that only the Liberator seems to have these certifications available... Is it because the Liberator is the only vehicle I spent certs on so far?
EDIT 2: The certifications are gone with today's update. Feel free to think that I'm crazy and imagining certs.