Separating Fresh Water From Pump Toilet

Thanks for the advice folks. I believe there is a check valve of some kind installed now. It's a large unit with a ball valve shut off attached to it designed for residential plumbing I suspect though I can't be exactly sure as there are no markings on it as far as I can tell. I should note that unlike modern trailers our water system is not under pressure at all times when the pump is on. It's old school. Of course the toilet is so old I can't find any info on it on the net.