Dc electical Problem

When my converter went bad it did the same think, the 12V voltage went way up.

Did you just replace the batteries? Sounds like you have two 12 volters in series.

How many batteries do you have? Are they six volt or twelve volt?

What is make and model of rig?
What is make and model of the converter? Some older converters had a filtered and non-filtered output.

First how are you reading the voltage ? Are you using a DC volts scale ??
As others have said, this is probably WAY off; knowing what kind of camper you have would help.

Can you ask the previous owner about this ?

What caused you to check it in the first place if "your electrical is all still working" ??

The second part of your question is WAY too broad for any kind of a meaningful answer. Maybe the fuses are supposed to be dead.

Have you had it apart?
Are the batteries installed?
Is this an inverter reading or battery ?
Is the inverter plugged in?

Funky meter? Low battery in meter? Test the meter. Connect to one battery and see what it reads or try it on your vehicle battery

26.4 is not correct. It should be closer to 12.6
Disconnect from shore power, NOW. It may even too late.
How many batteries do you have? If you have two they are not correctly connected together.
The positive of one battery should be connected to the other. The negative should be connected to the negative of the other.
The positive of one battery should be connect to the rig.
The negative of the other battery should be connected to ground.

You have a serious problem you should not be getting more than 14.5V MAX. Ideas, a converter which is failing two 12V batteries connected in series rather than parallel. Others may well come up with other ideas, I would disconnect until you find the problem.