Measure the voltage between the negative battery terminal and body ground. It should be zero. If you see 12 volts you have an open ground.
Some trailers of that age had a large fuse in the ground lead. Regardless, if there is voltage on that reading follow the negative lead to the point where it grounds to the body. If by chance there is a large cartridge fuse in a ceramic holder check the rivets that hold it together as well as the fuse
Rick Davis 1602 K8DOC
61 tradewind, plus a few others
13 Ram 2500 TD
99 Dodge TD 577K miles