What is this under my van?

When I was shopping for a new used van, I saw these on some of the ones with fiberglass tops. I figured that they had previously been wheelchair vans and it was a compressor or something for operating the lift.

But the van I ultimately bought has this thing, and it did not have a raised roof until I put one on. AFAIK, the van had one owner, who used it for towing a TT.

Like on the ex wheelchair vans with lift removed, on my van the bit that looks like an electrical junction box on a hose / cable is not attached to anything at the other end.

Whatever it is, can I remove it and put my batteries here, just behind the rear axle? There is, sadly, no space farther forward unless I go above the floor. Should I do that instead? If put them in the axle space, I will add more heat shielding on the adjacent exhaust pipe, could use advice about that, too!

Thanks you all, always.