Nice shot of a Rhodes 22 head compartment with a "real" marine toilet setup here:
Actually, while a lot of trailer sailors have portable toilets, a number of them were designed to allow a real marine head and holding tank. These were offered as optional equipment that many (most?) owners did not order with their boats. My previous boat, a 1977 Paceship PY23, never had anything but a porta potti, however, the porta potti sat on a raised shelf with clearance underneath to allow a real marine head to be mounted and easy access to the area under the V-berth for run hoses to a holding tank.
1973 Paceship Chance 32/28 "Westwind"