There is nothing wrong with remaining employed for the experience.
It's personal development, which is to be encouraged.
If I were you, I might also start looking for other opportunities that you might better suit you. Don't consider leaving until you have a written offer and are willing to accept it.
Also, think about what you can do to change the situation you've found yourself in.
If there's a proprietary tool / language being used, you could look into whether it would be possible to migrate away from it to a more standardized solution, or whether there are other innovative alternatives to it.
If you're there for the experience, spend your time focusing on personal development. Get any training, certifications, etc you believe will help you find your next position. This serves the double purpose of making you more valuable for future positions, but also gives you reason to pursue a higher salary at your current position.