Some are located in the crew's sleeping quarters (Revolving Toilets) and others in communal rest rooms. The restrooms ranged in various colors and style, depending location, and pre-or-post nanobot reconstruction. They were preaumably connected to the sewers of the ship.
When Lister and Rimmer decide to swap their bodies for 2 weeks so that Rimmer could have a physical presence and get Listers body in to shape, Rimmer takes Listers body to the toilet for the first time he decides to use a blind fold and tongs. (RD: Bodyswap)
When faced with being set back to factory settings by the nanobot-resurrected crew, Kryten forced the officers to go into a communal bathroom and remove their trousers, so that he could laugh at them. Counselor Lucas McClaren was seen to be wearing a female garter belt. (Back in the Red, Series VIII)
Captain Hollister was also seen spending a lot of time in the rest rooms during the series.
Whilst in the Tank when Lister and Rimmer were sharing a Cell they had to share a nonRevolving Toilet in plain view of each other in their cell. For privacy there was a blind fold provided so that others could not see who was using the toilet, however Rimmer got the use of this wrong when he went to use the toilet and put the blind fold on when in full view of Lister. Shocked Lister corrected him on this.
Years later, when the boys from the Dwarf once again had the ship to themselves, Rimmer ordered Kryten to gather up all the toilet rolls from the 2,143 restrooms of Red Dwarf. This was an attempt to move funds around and bribe the JMC On-Board Computer to not demote him. (Dear Dave, Series X)