Hi,
Welcome to the forum.
MotoJock:
Hey Dezi, My trailer sits for months in storage at a time. More often than not I have a problem with my lights, when I hitch the trailer to my pick up. I was going to make a light tester also, then I found this one online for a little more than it was going to cost me to build. Here's the web address - https://trailerlighttester.net
Good luck!
But that takes the fun away from building it yourself. 
Tom...