Newbie Saying Hi

Hello. I just joined this forum so that I can follow along
I have a 2002 Prius that I bought from the first owner when it was 5 years old and it had about 17,000 miles on it.
I now have 76,000 miles on it and lately, every few weeks at first-- now it's every few days-- all the warning lights are lit up! When it first started happening, I checked the gas cap, unscrew it and make sure it makes the "ratcheting" sound. I drive around town a few days with the warnings lit up and it drives ok. A few days later all the lights go off. I figured it was a vacuum or something simple. I read where moisture can get in the back through the vent and can cause all the lights. I read where an overfilled oil change could cause it (and recently had an oil change done as well.) All those warning lights and you'd think I immediately drive to the Toyota dealer. I'm ashamed of myself.
It does seem to happen after heavy rains and wind, or when the outside temps drop suddenly (it was in the low 20's the other day when the lights came on!)
I know I should take it to a dealer (nearest one is about 35 miles from here) but I'm afraid they'll tell me it's a $4,000 problem or some such awful thing and want to keep it for days on end to "fix" it