Advice on tear-down/rebuild?

Thanks Boonhauler. The design is intended to be 5th Wheel (front room is raised), but the outside looks like a box due to the basic floorplan software I was using. I am aware that the tongue weight should be a certain percentage and the weight needs to be distributed properly, but I haven't gotten as far as estimating and adjusting for that yet. This might explain why I find no floorplans similar to what I want :-) As for the plumbing, I haven't learned yet what my options are as far as placement, distance from fixtures, etc. I was hoping I could place them below the floor almost anywhere and run lines. No?

I know a structural engineer I hired once when renovating a quadraplex. (I thought the guy was going to make me pour enough concrete to build the Hoover dam.) Do you think the average structural engineer would be adequate for this, or is it a specialty?