This should be an easy repair, you will have to get a new valve, I would purchase online for best price. 3" pipe on the black tank and 1 1/2" on the gray tank side, standard valve. If you need other fittings you can get them at Home Depot, they will be white, but if you want black you will probably have to get them at a RV supply, probably 3 times the price. You can paint
them black. Do check and see if there is any other damage up where the pipes come from the two tanks down to the valve, if they where knocked off from hitting something there could be other damage that occurred at that time. Do test fit the pipes together before gluing as once glued it won't come undone again and you could be buying more parts to make the connection again.