Janet, (Sandy, UT)
Also, closer inspection will be necessary to verify the proper drain slope from the outlet fitting at that tank all the way to the termination point where you connect the sewer hose. Obviously water cannot flow uphill, so be sure you have plenty of drop for the entire distance. It will be necessary to remove the termination valves to replace the seals anyway, so carefully rinse and drain as much as you possibly can from each tank. Flush both tanks with fresh water as completely as possible beforehand.
Now might be a good time to upgrade your manually operated termination valves to electric gate valves such as those produced by Drainmaster. Uneven pulling on the slide valve by hand may have damaged the adapters on either side of each valve. Even if you don't upgrade to the Drainmaster, be sure the adapter fittings on each side of both valves line up properly and that the slide mechanism fully opens and closes. Any binding of the fittings and/or the valve and they might not close all the way.