Check your fuses. It sounds like the fuse for the converter may be blown and you may have some other fuses blown. I bought my travel trailer used and the guy had no idea why the 12v stuff wouldn't work. Turned out is was just the converter fuse blown. When you say nothing else works.....does the ac unit kick on? When you first put the battery in did the lights come on? If it's been a little while since you put the new battery in they it may very well be drained already. Put a battery charger on the battery and see if it is drained. I think it would be helpful if you could identify exactly was DOES and DOES NOT work appliance wise. It could be narrowed down pretty easily that way.
