From the sound of your question, it looks like you need either the Bristol Double RV Waste Valves - Elbow - Rotating # 34466ND14AAR (if you have 3" and 1.5" Hubs to 3" Spigot) or the # 34466NDHR19AAA (if you have 3" Hubs on both sides). Both of these models come with rotating valves and gates.
If you are looking for something a bit fancier and easy to use, that is the Valterra EZ Valve Double Electric Waste Valves - Rotating # E84-8. This comes with 3" hubs on both sides and functions... view full answer...