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Yes we have

But you're still asking a question that makes no sense. If you can install a 100A breaker in a 100A panel that breaker can carry the entire load of the bus. You just may but not be able to install breakers adjacent to it.

 You cannot add breakers. 

This is incorrect. This panel accepts twin breakers and there are a couple of positions where this can be done.

There is also another separate box labeled “camper”. We don’t use it so it’s turned off.

This would give you the opportunity for a space in the main panel. This breaker comes off a breaker in the main panel. And, yes, you can change circuits in the existing panel without completely "bringing it up to code".

The heating company has no idea what they are talking about.