vent cover and vent fan timer?

ScottG wrote:
You don't need a 12V timer, a 120V will do the job fine as long as it's the manual variety.
I used this one: Timer and just ran 12v through it.

That timer would work fine if the OP was in his RV all of the time, but he asked about it being in storage with no electrical power available. Since the timer you linked is spring wound and a 60 minute timer that would not work for him. I still feel that he needs a 12 volt timer for his situation. JMO

You also might want your vent fan on an adjustable thermostat so that it comes on whenever interior temperatures get high. Our RV's front vent came with a thermostat, so I leave it set on about 75 degrees all the time whenever the RV is in storage.

I guess the ultimate setup would be both a timer and a thermostat to guarantee some runtime every day, plus ensure constant running in hot weather to keep interior temperatures no higher than the outside air temperatures.

During the winter while our RV is in storage, we also leave an electric heater turned on inside and set to about 55 degrees to make sure that it doesn't get too damp and cold inside during wet and cold weather.

You don't need a 12V timer, a 120V will do the job fine as long as it's the manual variety.
I used this one: Timer and just ran 12v through it.

They do make 12 volt timers, but timers and fans do run off your battery and will deplete your battery unless you have a way to recharge it. JMO

With vent covers leave the vents open. If you have multiple vents leave one open at each end of the rig. This works well here in south Louisiana. Without hookup you would likely deplete a single battery in 10 days or so with what you propose.

I keep our vents opened year round, I have covers over them. Natural convection of heat rising should keep enough air moving. In three years I've never had a mould issue or bad smells. I do leave a couple dryer sheets laying out to have that nice Downy smell when we open the door.