[sourdough]

Installing a check valve on the hwh won't have anything to do with getting cold water to the rest of the RV. It has to be something in the feed line. Is water on? Do you have flow at the inlet on the outside? All faucets no cold water? Toilet no water? If no, check where your cold water comes into the RV inside the wall at the inlet port - do you have water there? (Yes, you will probably need a bowl to catch any water).

If you've lost all cold water for some reason it won't have to do with the check valve. It sounds like you installed it correctly (arrow pointing away from the hwh). If you can't get water to the faucets or toilet it will do you no good to pop the pressure valve on the hwh to try to fill it - it sounds like you lost the water somewhere upstream.