I am treated very fairly where I work. When it's time to "low census" someone, my name goes in the hat with the FTE's. We all take turns, so it's not always ME getting called off. We staff creatively, also, in that we will split it among us. Sometimes we each will get off the clock for a couple of hours, remaining "in house" (taking a break) and each only loses a few hours that way, versus a whole shift. I also am lucky in that I am regularly scheduled a weekend night every week. So I work every week, at least one shift, unless there is a low census situation and it's my turn to go.
In return, I am very willing to cover sick calls, vacations, and switch with people who need nights off for various reasons. I guess this is why they work with me and treat me so well. I feel very fortunate. I stay on as "per diem" also, because, like you said, I do gain seniority along with the FTE's. It's a good deal I have.
Look around and see, maybe you can take another perdiem job (and keep this one). Nothing saying you can't have two of them, if you are not getting enough hours where you are now. I had to do that for a while when things were extremely slow and I was getting called off constantly. I think they saw I was able and willing to work elsewhere and I was. I had no qualms about taking care of myself. I give and I get a little. I have a very good working relationship with the people at my unit. They will work with me and that is a real plus.
And it does not hurt me that I work for a unionized hospital, so no rules are changed without notice for us.