Where in the Bible does it say "scripture only?" It doesn't, and I have not said that. However, I have cited 2 Tim. 3:16-17 on the authority of scripture coming from God. You can't cite such a thing for extra-Biblical traditions.
Trinity. Yes, I do: that's a man-conceived phrase for a plain teaching in scripture - that there are 3 persons in the God-head. I don't think I have to cite the basis of that for you. But is this an 'extra-Biblical' tradition (pre-empting the next question)? I say no, because I can literally cite chapter and verse the basis of this doctrine.
Christian Unity. We Protestants indeed fail here ... every denomination created after the Reformation has been created on the basis of either error, style, emphasis or some combination of all three. Most Many of us have certain 'essentials' that we agree upon. Of course, if this is our only problem (unfortunately not), then that's pretty good. Clearly, God wants us all to get it right... which leads into...
Truth. Well, there's the rub, right? Yeah.... we need to find the truth. I'm hardly perfect, so no, I don't claim to know it all. I'd like to think I recognize obvious error when I see it, and my complaint with the RCC is that a sizable chunk of your doctrine is not backed up by scripture. Maybe you don't understand it from an outsider's point of view, but saying "well, we do it this way because we always have believe it to be right" doesn't have the same strength as "we do it this way because it's commanded by scripture that I can show you right here." That's the essence of my complaint.
The trunk. You are correct - the RCC is the trunk of the church. I recognize that's a hard thing to question or to give up...Especially when you see a myriad of competing options out there. My goal here is not to get anyone to join my church. It's to recognize (maybe) that scripture has much more weight in matters of doctrine than anything else.