I would think that moving the commode back would be a relatively big project. In addition to having to move the water and drain pipes, you would have to fill the existing hole in the floor and reinforce it. I don't know what would be involved with cutting the tub, but I would think that would also be a big project to get it so it looked good and did not leak.
The following is not knocking you, but advice to all people looking at an RV to buy. The best thing you can do is go through all the motions of daily living in the RV before buying. Get in the shower and go through the motions of washing. Sit on the commode and go through the motions of that activity. I don't know how many RV's we looked at where I bumped my head when trying to wipe.....
Go through the motions of brushing your teeth to see if there is enough room. In the kitchen, go through the motions of preparing a meal and the cleanup after. Actually doing the activities will show up faults in the design.