Uh, what you are describing is done sometimes to power items on the same circuit in emergency situations but it is not a good practice. You can burn up wiring depending on the generator- if over 15 amps output--too much. It would have to be 2000 watt or less. You also have to turn off all circuits but the one that is on that outlet as the feedback would try to power everything on every circuit- messy.
Another mistake made on generators is thinking that you get more power if you use a two outlet adapter to single output. These adapters were taken off the market not long ago as people were blowing house circuits and ruining things. This is dangerous. Smaller generators may only have a two standard output receptacle. (120V dual wall outlet style). Use one and only one outlet to hook up as they may not be in phase. The generator will give all of its available power to that one outlet based on load.
Everyone already covered most everything. Always use the trailer power cord to plug it to the main power coupling on the trailer using the adapter at the generator end or wall socket/power source. I have one of those circuit protection devices that I use. It has a current draw meter on it. My AC pulls 12A at initial start then goes down to where the meter alternates between 9-10 Amps if you watch it long enough. (Dometic Penguin 11K BTU unit). I always have a 50/30, 30/20(120V) adapter set for my 30AMP model and this optional piece. I keep it attached to the trailer power cord at all times with the lock cap. It protects against just about everything- messed up ground, too low a voltage for the draw, surge, etc. It runs a series of tests before completing the circuit. I find it takes all the guesswork and wonderment out of electric and protects the trailer too:
2015 Ford F-150 3.5 Ecobeast 3.55 Lariat