No problem plugging the fridge directly into the 30 amp service, with the right plug adapters, of course. Should also be no problem plugging it into the 15 amp plug on your trailer, which is then plugged into the 30 amp service.
Both that small fridge and the RV's AC unit will pull something like 12 to 14 amps when the compressor first starts, for maybe 2 or 3 seconds. Otherwise, they will only draw around 5 or 6 amps while running.
Since the compressor startup only happens for a few seconds every 5 to 60 minutes, but sorta randomly, the odds of both kicking on at once are slim. However, do be aware that although unlikely, it is possible, and for that 2 or 3 seconds, there won't be much capacity for running much of anything else in the RV. End result, at worst, you might have to go reset the breaker. Expect to have to do THAT once every couple years or so. And if/when it does happen, remember to unplug the fridge before you reset the breaker so that both AC & fridge don't try to come on at the same time when power is restored. Remember to plug the fridge back in on your way back into the RV.
