I think the general SWAG is about a 50% duty cycle at normal air temps (75F-ish) and that they typically use a bit less than their rated current at normal temps, so think about 3 amp-hrs per clock hour (as a conservative estimate.) That's what I'd use to size my battery bank. I was planning 2 100AH AGMs for the 28' project, and figuring that would be enough overkill that it would let me not have to be obsessive about managing it. With some portable solar and the low electrical demands of a vintage trailer you could probably do fine on sunny days w/ that 80AH battery but you'll be deep-cycling it every night.
Now, I'd probably still choose the AGMs (because I'm a cheapskate) but there's some slick LiFePO4 stuff out there that I love everything about except the prices... Light, sealed, durable... AGM still has LOTS of bang for the buck.
And yes, unless you've really drawn down the trailer's battery before hitching up, I'd think the truck's factory 12V
charge line would provide plenty to run the fridge and slow-charge the trailer battery.