Anything is possible with enough time and funds, the question is do you want it bad enough? As mentioned, you would have to move the black tank forward so it would sit under the toilet in its new location. That means you have to either run the discharge pipe all the way back to where the discharge outlet is or relocate the outlet nearer to the tank. And if you are working over the wheel wells, keep in mind that the axles are below and that may complicate working around them unless you drop them first. Here is what you might see once you pull down the belly pan and old insulation:
You also need to think about where the vent for that tank is and figure out the plumbing for that as well. You may be able to leave the gray tank where it is and just run the drain line back aft or you may want to move it, hard to say without seeing what the plumbing looks like. once you pull out the sink and bathtub, you'll have a bit of piping to deal with. Check out my blog if you want to see what I encountered in a similar vintage rear bath Overlander. The hot and cold pipes are simple enough and you will probably want to convert to PEX for cost and ease of installation.
And don't forget the hot water heater, its probably located all the way aft as well, so if you move it you need to cut a new hole in the side and then patch the old one. You would also need to move the gas line too. If you don't move it, then you would need to figure out how to leave it where it sits and work your new configuration around it.
And you will probably want to move the bulkhead wall that defines the bathroom. It may have the same contour and can just be slid foward or you may have to make a new one. On my trailer, on of the vent pipes went inside that wall so you may have to relocate it or find another way to hide it.
If you were going to gut the trailer and redesign the interior, it would probably be a simpler task, when you try and remodel and reuse existing walls and cabinets, it gets a bit tricky. DAMHIKT