I had same thing happen. All 12volt went out. Few hours later came back on. 2 weekend trips later, same thing. 2001 XL 390 Power block in engine compartment (under bed) main 12 volt
line to series of breakers. Dust and some corrosion just enough to kill power. Replaced one breaker, cleaned others. A little battery protectant spray. Fixed. It was frustrating / perplexing to track down, especially when it comes back on as randomly as it went off. This would be a classic, 'I should clean that up before it is a problem at bedtime' issue. So stupid simple, but if you had to pay for diagnostic... Especially when it comes and goes.
Sometimes you just get lucky.