Here is a greenhouse designed for TAC life support, based on the stock KSP cupola model. It converts Electricity (lots of it!), Waste, Carbon Dioxide and Waste Water to Food. There is some loss in this process, so the crew will still need to be supplied.
Growing food in a cramped greenhouse in zero G is very energy intensive. One Mk1 greenhouse uses about 1.4 Electrical Units per second, or about 5000 per hour (the RTG produces about 0.75/second, so two RTGs could supply one Mk1 green