12V system not working...

ScottG wrote:

ronwalt wrote:
Please allow me to post the back story in case any of that has anything to do with current issue. Went to hook up trailer for camping trip. Power tounge jack and slide out not working despite being hooked to 120V. Single tongue mounted battery has has 3.5V and 12V system not working without 120V. Soon discover that power jack and slide out needs good battery regardless of 120V available. Install new battery and all is good. At campground they have a power outage for maintenance and the 12V system isn't powering all 12V systems except for power tongue jack and slideout working fine. No 12V lights, radio, or fridge. Battery voltage is still 12.5V All fuses good; 2 30amp. mounted near battery, 2 40 amps behind converter door and several 15 amps....all good. Battery cutoff switch is in ON position. And finally, all 12V systems work perfectly when plugged into tow vehicle. Any info. on possible causes or areas that should be checked are greatly appreciated...Thank You!

I'm a little confused :)
Early in your post you say battery voltage is 3.5V then later you say it is "still 12.5V". Sounds like the battery isn't hooked up. Or maybe you hooked it up backwards and blew the reverse polarity fuses.
Assuming your RV battery was at 3.5V and connected, I would say your converter has failed .
When you plugged into the tow vehicle, it brought the voltage up.
Sorry to say, it's time to replace the converter.