It's not the batteries.
If it were the batteries, you'd get some 12V power, but it would fade quickly. For example, you'd turn on the lights and they would come on bright but get dimmer and dimmer over the course of a minute or so.
With zero, zip, zilch, no 12V power at all, something is disconnected, a fuse is blown, or a breaker has tripped and not reset.
It is probably the main breaker between the 12V batteries and the converter. Every RV has one. It is usually buried behind the batteries or in some out-of-the-way place. Follow the red wire off the + on the batteries, and you should find a black box with two terminals on it. That is the main breaker.
They often go bad, but sometimes they are trying to tell you that you have a major problem. If it has a reset, try resetting it. You could jumper it with a screwdriver for a short time just to see if you get power. You may need to replace the breaker.
