The Centurion may not have a relay. You get 12v ok from converter, and your battery is charged, so your problem is between battery and DC dist panel.
The flare up and then dead symptom could mean some sort of bad connection. Look for the connections at the battery post, the battery catastrophic fuse, the battery disconnect switch, and the battery wire lug on the DC dist panel.
1. 1991 Oakland 28DB Class C
on Ford E350-460-7.5 Gas EFI
Photo in Profile
2. 1991 Bighorn 9.5ft Truck Camper on 2003 Chev 2500HD 6.0 Gas
See Profile for Electronic set-ups for 1. and 2.