Water damage is the biggest enemy of trailers. You DO NOT want to wait to find out you have a leak before you do something. It will be too late. The damage will be costly.
To protect your trailer you have several choices and the main objective is to keep water off your trailer.
1- Keep it in a garage.
2- Keep it under a tarp.
3- Cover every horizontal seem with 100% silicone. Silicone is like liquid metal. It WILL make a mess. It will attract dirt and the only way to get it off is with a grinder. But, nothing will stick to it and it will protect your trailer.

You decide what to use.

Keep your trailer clean. Dirt is a good place for mold and mildew to live and grow. Get a good soft and scrub brush and wash it once a year.

Don't drive on wet roads. Most vintage trailers have poorly protected bellies. Try to keep them dry by not driving on wet roads.

Don't hit bumps or drive fast on rough roads.
The frames on these trailers weren't that strong. They can be bent easily.

Drain all the water from the lines and the tank before a freeze. Frozen water will crack anything it's trapped inside of.