The cable sheath could be the issue. You might try PB Blaster, a penetrating oil that works great on cables. It's available at Home Depot, auto parts stores, etc. A step up from WD 40 for this type of job.
If you can't loosen the cable pull, you might have to replace the cable and gate valve. Since you have an enclosed cover, you might have to remove or cut through that to access the valve area. You might check with manufacturer or RV service dept. to see if they have a least destructive method to do the job.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI