I'll second aftermath's statement... it's only necessary to get it approximately level, perfection is pointless (and I'm obsessive about lots of pointless things!)
There's no standard for a relationship between marks on the stick-on bubble levels and degrees or inches off-level. The trailers all came from the factory without those, and there are many brands of leveling devices. Just get it where round things don't roll around much on the floor, the cabinet doors stay open (if you have any... '79 would be a tambour interior I guess) and the bacon grease doesn't all flow to one side of the skillet.
Remember, every minute you waste perfecting the level could be more productively spent showing off your trailer to an admiring neighbor or having an Adult Beverage!