First I would like to thank you all for your reply
Looks like I have found the problem, if its a problem....
I could reproduce it at will, all I have to do is to close the main air outlet under the ac while leaving all the other registry open , then set the A.C in the trailer at temperature outside the trailer and wait , ( doing that at night )
It wont take long for the AC to start, to run 1mn , and to shut down. The same cycle start all over again and I waited for 6 of these cycles to confirm that it is not a normal sequence of some sort.
Then to stop this all I had to do is to open the main outlet under the A.C... no more problem to report, ac resume to a perfectly normal operation and cool the trailer pretty well.
So, now I need to find someone to tell me why this is happening but only when the outside temp and inside temps are almost identical. It is some kind of protection ? Any way glad to have resumed to a normal operation during the night...its absolutly annoying to hear the ac startiong and stopping every 10mn or so