We did not
paint our floor first. Our installer (a professional certified installer I use on my commercial interior design projects)
said he could not apply over a painted surface. The glue needs to penetrate the substrate to adhere properly. If it was painted, he would have to sand it down before he could apply the Marmoleum.
I know others that will disagree with me, but we have installed sheet Marmoleum in two Airstreams and never had any problems. Water does not soak through it and get to the plywood - the glue seals the floor & adheres the flooring.
Besides, every screw or tack or nail that holds your cabinetry to the floor breaches the "seal" that people try to achieve with paint anyways.