Almost all of my camping for the last 15 years has been off grid in the Southern California deserts. Take my suggestions for what they are. Just my opinions from many years of doing what you describe.
If you are just charging phones, tablets and a laptop a 300 watt inverter Is really all you need.
If you have or want a larger inverter to power all the AC outlets; I would suggest finding a place to put the inverter where you can just use a 120v adapter and plug your shore power cord into the inverter. BUT I speak from from experience here. Do not forget to flip off the breaker on your built in battery charger or you will wind with dead batteries pretty quickly. Don't forget to take the refrigerator off Auto as it will suck the batteries. If your water heater has an AC option make sure it is off as well (Been there done that, I'm apparently a slow learner).
Also be aware that the circuit breaker for the battery charger might also power other outlets in the trailer. In my trailer when I flip that breaker off I also lose my entertainment system (TV, DVD player etc) which is not really desirable.
While both systems above work, they have losses as you convert from 12 volt to 120 volt and back to 12 or 5 volt to charge the phones and such. What I found to be most efficient is to install a bunch of 5v outlets in the trailer. I now have one outlet by each bed for the kids and one on each side or my queen bed as well as one under the dinette table and one on the side of the kitchen cabinet. All told I have added six 12 volt outlets. This allows everybody to keep their electronics charged without going through the battery losses that come with inverting and converting. For items that cannot run on the 12v outlets I just have a small 300w inverter but I rarely ever use it.
Here's a few pictures of what I'm talking about. If you go this route make sure you get at least 2.1 amp ports. And that is 2.1 amp each port as many of them are 2.1 amp on one side and 1 amp on the other.
I also had to look for a while to find non-illuminated ports as I dislike the light from an LED in the trailer at night out in the desert and also don't want the vampiric power drain of even an LED.
Amazon link
