I use a portable 30 amp surge protector that looks like this:
I got it at Camping World
I also use an inexpensive volt meter, which looks like this:
Also from Camping World
The surge protector is more to protect the trailer from surges & lightning.
I use the volt meter to assure the voltage is sufficient to operate the A/C.
Rule of thumb I use is to make sure voltage is above 104V before I operate the A/C. Below that you're at risk. Below 100V you're in danger blowing the motor in your A/C.
I also use a circuit tester before plugging in the trailer to assure the campground's shore power is wired up correctly.
Here's what the circuit test looks like:
You can pick one up at most hardware stores.
This is what we do.
IMO the autoformer sounds nice, but it's way too pricy for me.
I don't know anyone that has one. So I can't provide any comments on them.
Hope this helps.