I think the fabric underneath is throwing you a curve ball. The only thing hiding between that fabric and the plywood floor is a small bit of insulation.
Please correct me if im way off.
I would use an existing hole or drill a new one. What are you thinking like a 1/4" tube? You could probably peel away the expanding foam from an existing hole and stuff a tube down. Then reseal.
Or make your own small hole. Start inside the trailer and determine a good area. Then go under the trailer to see where tanks and metal frame are located.
I recently drilled a 4" hole in my floor. Spent many many hours debating where to put it. Scared to actually make the cut. Once it was done i realize that i stressed over something that was very very simple. Youre drilling a hole though plywood and can see above and below the plywood. Not really any hidden variable to screw you up.