Didn't all the Old timers have fixed open vents on their trailers... A trick we used on Kustom Vans long ago , was to put a small vent (s) in the floor too ( with cover... boat parts) ... which when used with a top or side vent ( or cracked open window ) , created a positive airflow ... Warm air rising and exiting the higher vent , cool fresh air entering from the bottom ... Now one of the more scientifically inclined among us , can calculate the airflow through a (X) diameter vent and determine the flow rate/vent size sufficient to support life inside a trailer... only variable is how many cows are going to be in the barn ....
And I have to Totally Agree with Auntie on the good sleeping bag thing ...
Good bag and you're ready for most anything ... Doesn't everybody have sleeping bags left over from the "tent camping" days ???