ᐅ Camping Cabin Floor

Hello
I bought a house on a campground and got an unpleasant surprise. When I removed the partition walls and laminate flooring, I broke through the floor. Everything was damp and rotten. Now I want to rebuild the floor. Which materials should I use?

1: I want to start with foundation blocks.
2: Which square timber (and what size) can I use that won’t rot from moisture?
3: Which floorboards (width, thickness) can I use so that I don’t break through again and to prevent dampness and mold?
4: Which wood can I use for cladding the house that resists weather and moisture?

I look forward to many answers and thank you in advance.

Well, as described above, I want to redo the floor.

I got 4 squared timber beams, each 4 m (13 ft) long.

Foundation stones and OSB panels with a thickness of 22 mm (7/8 inch).

Now my question:
The building is 5 x 7 m (16 x 23 ft), and the 4 timber beams are only 4 m (13 ft) long.
So I have to splice the beams and cut them to size.

What is the best way to join them?

Because it is damp, they should not rot or have any other damage.