50 amp to 30 amp adapter, only half of the electrical works

[BobAndLinda]

We have a 2018 25RES Cougar 5th wheel that we have traveled with all over the country. It has 50 amp service, but we generally use 30 amp because that is all we need and have never had a problem. However, at this campground, we connected to a 30 amp service outlet and suddenly most of our outlets don't work. The AC and water heater won't work either. However, one outlet, the microwave and a few other things work. Is it our adapter? Or could it be the campground plug?

Thanks!

[travelin texans]

Hopefully you have a surge protector/EMS being used also!! If you're still traveling across the country I wouldn't plug in anywhere, 30 or 50 amp, without one.
Back to your issue, try plugging into the 50 amp plug with your 50 amp cord that should tell you quickly if it's yours or theirs if you don't have a VOM meter to test both.
You said you don't need 50 amps, is it that you don't want to lug around the 50 amp cord? That's the only advantage I could think for doing it your way.
When I had a 30 amp RV I did just opposite, I'd use the adapter to plug my 30 amp into the 50.
If you've been in the southwest later than April you would need 50 amps to run at least 2 acs, 1 will never git'r done.

[Just chillin]

Your first thought is a bad adapter. It feeds both legs of a 50 amp and sounds like it may have lost a leg.