When I had an Overlander (1961) I wanted the same idea. I looked at generic RV step tubs to sit up on the floor and step up over the wheel well but I eventually axed the plan and went rear bath. With vintage width it's tricky but doable. I think the key may be accepting that the side bath will eat up a lot of the available width and that whatever is on the opposite side would need to be quite narrow, close to 12" really.
1960 Sovereign 33' Pacific Railroad Custom