Welcome to the forums. The wires are probably larger because the Univolt has a larger capacity, along with the possible realization the wires should be bigger in your trailer.
It would be a good idea to get rid of both Univolts, and get a new one. They're not that expensive, are quieter (the 1970's Univolt still hums), more reliable, and treat your trailer's battery better.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.