It looks like the red arrow is pointing at the water tank and that there is framing above the water tank supporting the floor.
Instead of removing the floor, what about instead creating access on the side, below the framing and below the box upper surface covering the top of the water tank.
My speculation would be to expose the side opposite the red arrow, probably near the down slope. That looks like where the vent and where the wiring exit that volume.
There is probably enough distance between the top of the tank and that enclosure surface to permit working on those connections.
