Flooring

Haven't opened the membrane under the camper, was hoping not to have to do that so I don't have to repair that.
I'll get more into it the next few days, just didn't want to cut out the section to find nothing underneath I could brace the new section too.

Have you looked up at the underside of the floor area in question from the ground? You might be able to see what sort of framing is in the area that way, or at least get a reasonable idea of what you're up against.

I would guess that both the floor and the steps are attached to the same metal trailer framing members. I could also very easily be guessing wrongly.