Tiny on a trailer

I just bought this old travel trailer frame. It's 22x8. Planning to make a small 8x6 porch on the tongue end and dry-in the rest with a "barn" roof with a small loft. I will then pull it back in the woods to use as a getaway cabin. I have built a few buildings, including my house and place of business, but have never built anything on a trailer before. In my sketch, I have 2x4's laid crossways on the trailer, anchored to the outriggers, for floor joists. Why 2x4's? For weight and height considerations, and because it seems that is the way the floor might have originally been built. They only need to span about 5'9'' between those main I-beams and would be laid 16'' on center with 3/4'' plywood over that. If that's wrong I would like some input on that. Because I'm not aware of how the stresses of pulling this thing down the road (if I need to do that someday) would affect this. I'm also considering using pressure treated for the joists and plywood because we have carpenter bees here. Thanks

I built one on a 20 foot trailer about a dozen years ago. I used treated 2x4's  16 on center. I laid down half inch treated ply on the trailer then the 2x4s then filled the bays with foam insulation and laid 3/4 inch ply on top. Then lined up the deck to the trailer and ran 3/8 inch bolts through the deck to the perimeter of the trailer every 32 inches. I glued and screwed all the sheeting to the joists and studs.

You are wise to keep weight in mind. Find out the weight rating on your trailer! Mine was right at 7000 pounds for just the  shell including electric and interior wall sheeting.
 It handled a 500 mile road trip just fine. Including the last 2 very steep and rough miles.

Nice project!
If you are going to make a porch on the tongue end, it would seem like a good plan to put a platform on the lower triangle part to use as steps down to the ground, concealing the metal frame as a side benefit.