What size and volt do you use to change tires?

Milwakee or Dewalt, biggest you can find.

if you have a generator on your trailer a plug in does the same for much cheaper. 

Hmm, I didn't even think about using the generator.

We have a 20V Dewalt with a 5ah battery in both trucks. Every couple of months I check the batteries to make sure they are still charged.

We got the socket sets at Harbor freight. Make sure you get the long ones. 

This is the one we have:

https://www.amazon.com/DEWALT-DCF899B-Brushless-Torque-Impact/dp/B00WJA1J68/ref=sr_1_2?dchild=1&keywords=dewalt+impact+wrench&qid=1622753808&sr=8-2