Help re' cutting into trailer for electric input

I've been reading that folks often use a hole saw to cut the round opening in the side of the trailer for electrical inlets and other inlets.
My question is, is it better to cut through the skin only, or to have the insulation and inner plywood in place when the hole is cut? I'm thinking that the extra material behind the skin might stabilize it and make for an easier cut, but, on the other hand, cutting through several layers could possibly make the cutting more difficult. I used the sheets of styrofoam-type insulation.
Which have you tried, and what would you all recommend? This will be my first cut into the skin, and I'm nervous.
Thanks!
Robin
My question is, is it better to cut through the skin only, or to have the insulation and inner plywood in place when the hole is cut? I'm thinking that the extra material behind the skin might stabilize it and make for an easier cut, but, on the other hand, cutting through several layers could possibly make the cutting more difficult. I used the sheets of styrofoam-type insulation.
Which have you tried, and what would you all recommend? This will be my first cut into the skin, and I'm nervous.

Thanks!
Robin