Use a battery charger. You can test each circuit without fear of blowing things up. Just connect the black lead of the charger to a clean spot on the skin or frame. Then use the red lead to connect to each circuit you want to test. This applies to the DC circuits. It is much safer to use a charger than to use a battery since the charger has a limited amount of current capacity. Whereas a battery has an infinite amount.
With the AC circuits, you have circuit breaker protection. Usually there are only a few circuits. Outlets, converter power and Air Conditioning.
Obviously all wiring should appear to be in good shape, with the appropriate fasteners entering boxes etc..
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