I do feel compelled to point out that performing valuable work on a volunteer basis (or nearly volunteer) is not helping your cause. If you feel disrespected by your full-time colleagues, it's partly because your efforts serve to devalue their ministry as well as your own.
In their minds, they have to resolve the disconnect; why do I get paid $xx,xxx for doing what Roy does in exchange for $x,xxx (or $0)? The easiest and most likely way that they answer the dilemma is by concluding that the content and/or kingdom value of your ministry is not equivalent and thus does not merit the same $xx,xxx in compensation. Is it then hard to understand why your full-time colleagues don't hold you in high esteem?
You could say that your reward in heaven and the satisfaction of serving the people of God is more than enough compensation, but your post seems to reflect that you are not satisfied with the part-time arrangement. You're absolutely entitled to feel this way, but don't be surprised if some people see it differently.