I thought we had a similar problem. We bought a used TT and when we went to check it out the owner gave a very thorough walk thru. We tested everything but the AC. He said that he did not have a 30 amp plug-in at his house but "promised" it worked great. We got set up in a park plugged in and went in and turned on the AC it blew hot air. I let it go for ten minutes and it still was not cooling. I was so upset that I bought a TT and did not check it out completely.
After a few more minutes I walked over to the vent and said cautiously to my lovely I think it is starting to blow cooler air.
Long story short the inside of the trailer was so hot that it took a lot longer for the vents to start blowing cold air it seemed like forever. It probably wasn't that long but when I figured it was my fault for not pre-running it just seemed longer.
It did a great job of cooling after that.
I hope you get your problem fixed and get on the road.
jesse