Bad battery or possibly a bad converter. FWIW I had a converter go out on my trailer that was only 16 months old. I replaced it with a better grade of 4 stage.
As Marty said; you don't really want to run the 12 volt stuff without a battery in the circuit. It can be done, but will cause problems if you do it on a regular basis.
RV's take a fair bit of fiddling and maintaining to keep them in top notch shape. There are a lot of systems to stay on top of. When you figure that you basically have a small house that is going through a hurricane and an earthquake every time you tow it down the road they do pretty well to stay together as well as they do.
I have a mental checklist (need to get it on paper) that I go through once a month when the trailer is parked, as well as a slightly different one that gets used prior to leaving out for a trip.
A battery disconnect would not be a bad idea.
Check the date on your old battery and see when it was made. If it is over 3 years old it was probably on it's last legs. Five years is pretty good for any battery, RV batteries typically only last a couple of years unless they have had extra care.