I have a 2008 Designer 35' and earlier this year I had a similar problem. After checking the batteries and finding them full of water. I looked for other issues.....the converter had to be the culprit.
I was plugged into shore power and the outlet plugs 110v worked great....but anything that was 12v was working correctly. So I did what I thought was the obvious.....I found the converter....and unplugged it from the 110 v outlet....waited about 5 seconds and plugged it back in......and guess what.
Everything worked just find. I sorta thought of it as a computer just had to be rebooted. Probably not correct...but what I did.\
This was about 60 days ago.....and have been out a couple times since.
I have since used the trailer for both boondocking....and plugged into shore power. Thankfully No problems since. Not sure if I am lucky or just have not really found the real problem. But for now.....I am happy.
"Can't wait to have more fun"
2007 Dodge 3500 Dually
2008 Jayco 35RLSA