In at least some RVs, the tanks are put in before the floor is laid, and cannot readily be removed and replaced without removing the floor.  I have no idea if this unit is built in that way or not.  I would not expect it to be a simple straightforward job regardless.  At any rate, you could get a better answer than most of us could possibly provide by examining the unit and seeing how the tanks are mounted and/or enclosed underneath.
I'm also unclear on why you'd want a single holding tank.  You won't gain a significant amount of capacity.  It would be a lot easier and simpler and less expensive to install a pump to transfer gray water to the black water tank if you need to equalize them.  (Even if the only gray water you use is for handwashing after using the toilet, I doubt you'd be too likely to fill up the black water tank long before the gray water unless the bathroom sink is plumbed to the blackwater tank—which is sometimes done.)