Granvia wire up cabin fan to 2nd battery. Good or bad?

[dmt_surfsup]

Hi all. I have 2002 Granvia (Grand Hiace) Japanese import. I'm looking to convert it into a camper van for weekend surf trips. I have 2 questions, firstly is the onboard factory fan economical enough to run overnight on a second Battery system? Not using AC, just utilising the existing fans that blow air into the cabin from the roof. Was thinking to add a switch between the batteries. Secondly if you did wire it up is it possible to add an option to reverse the polarity and use it as an exhaust? I'm just curious if this is a go or no go. The next option is to cut a hole in the roof for an electric vent. I'm based in Brisbane where it's hot and humid so a vent of some sort is a must. I've attached a photo of the vent I'm taking about (not the sunroof 😃) Need airflow and mosquito protection.