I understand what they are doing. What they are hoping for is something that was unlikely to have been kept as a long term record. What they want to know is the diameter of the hole that was made at the factory for installing the original electrical
cord port. Most people just remove the old part and then they try to find a suitable replacement rather than the other way around. If they wrote that measurement down it was likely done on a piece of scrap paper than was tossed away shortly afterwards.
Or you could get creative with cardboard and tape and make a poor man's caliper to measure the size of the hole from inside the trailer. A piece of cardboard on each side of the hole and one crosswise, tape the pieces of cardboard together, remove cardboard as one piece and then measure the gap. That will tell you approximately what diameter the hole is.
Every elementary school student should be required to learn how to measure with calipers. I teach 3rd graders how to use them. They are so easy to use for this kind of thing. Every DIY tool box should contain this extremely handy tool. If you don't know how to use one here is a tutorial I wrote on it several years ago for a group of people who make miniatures but are not too familiar with using tools.
If you had a pair you could go right out and measure the round shape that is inside of the hole that projects inside of your trailer and that would tell you the hole must be at least that large if not just a little bit bigger. Then you could measure the replacement you bought and see if it is a close match. Your local hardware store is very likely to have an affordable digital caliper in stock. They have become quite common on the tool aisle in the last couple of years.