Champion 3500/4000 Generator won't run AC unit?

Had your 5th wheel been plugged into shore power up until the Genny test?
If your batteries were low on the 5er the converter would have been trying to recharge, possibly at a high rate depending on the converter you have. Also you may have had the refrigerator trying to start at the same time.
Try turning the converter and everything else off and starting the AC then and if that works, after it has setteled in to run the AC turn the converter back on.