Are your valves operated by a steel cable? I had the same issue. I ended up by removing the cables on all three of my tank valves and was able to drill a small hole through the frame and I installed long extensions for the valve stems.
Another thing you could do like many of us do is install a twist on valve on the end of your discharge pipe. Then mount the cap on that. This serves as a back up valve in case one of your tank valves leak a little. When you're ready to dump, remove the cap, connect your sewer hose, then open that valve. Any leakage behind that that valve will indicate a tank valve leaking by a little. You can also close that valve, open up the tank valve that's leaking and 1/2 fill the tank with water. Let is sit for a few days. This may dissolve anything in the valve track. You can also add a septic tank cleaner.
https://www.amazon.com/Valterra-T58-Twist-On-Waste-Valve/dp/B000BGHYJS
https://www.amazon.com/Roebic-Laboratories-Septic-Treatment-32-Ounce/dp/B000BQU7IS/ref=sr_1_4?gclid=EAIaIQobChMIz973j_7Q4wIVBZ-fCh0KrgrkEAAYAiAAEgLOwvD_BwE&hvadid=313632841677&hvdev=c&hvlocphy=9002488&hvnetw=g&hvpos=1t2&hvqmt=b&hvrand=3107639307519893883&hvtargid=kwd-331092180625&hydadcr=21445_9712267&keywords=septic+tank+treatment+roebic&qid=1564089152&s=gateway&sr=8-4
