I didn't see anyone mention the battery disconnect yet. If you have one, it might be in the 'disconnect' position. In my TT, what you describe is how my behaves when the disconnect is hooked up. When plugged in, my 12v items are powered by the converter. When hookup to the truck, they're powered by the truck. But if not hooked up to either, then I have to have the battery disconnect in the 'connected' position to have power.
On mine, it's a plunger style. In is disconnected, out is connected. It helps to think of it like one of those big red emergency stop buttons. Punch it in to kill the power, pull it out to reset.
If that's ok, the next thing to check is the current limit and reverse polarity fuses. If the battery got hooked up backwards at any point, it would have blown the reverse polarity fuses, and even if you turn the connections around it still doesn't work. Check those first, since they're usually the easiest to get to.
After that is the current limit. This could be a fuse or a resetable breaker, usually withing a few feet of the battery. In most cases if you lay down under the tongue and trace the leads from the battery they can be located. They'll go from the battery to the limit/fuse before going to anything else. Make sure they're good. The fuse is just a matter of replacing, the resetable limit will have a little button on the side of the box. If it's out, it's tripped and needs to be pushed back in.
So, all in all with the other recommendations on here you've got:
1) Battery disconnect
2) Reverse Polarity Fuses
3) Current Limiter
4) Top Up/Down switch
5) Sink Up/Down Switch
Check those and let us know.
There's no use crying over spilled milk, unless it's on your keyboard.