Not familiar with the drain layout of the trailer and online drawings are not there for trailer group, so this has to be something closer to theory than actual knowledge?
I would first want to gain some more ideas of what the problem might be. First I would want to sort the problem down to whether it is one tank or both and then begin to look at what is happening.
To sort which and how many, I would connect to water and add a good amount of water in first one then the other with the valves closed. Do you get much the same leaking from each?
That would seem to indicate needing more work or repair than if only one is leaking.
Then a big question comes up. How far is it from the outlet end to these valves? Is it practical to try to spot the problem by going in at the outer end of the drain pipe to get a look at the valve insides by using a long scope/camera on a ther to a smart phone?
If working a short distance, this might be easier than if a long space to work past.
Maybe something as cheap as this for one time use?
https://www.ebay.com/itm/39318155286...Bk9SR_yItrSHYg
This would let you look for problems like junk stuck in the groove where the valve should slide into and make a good seal, hunks of the slide broken or damage? It might also let you judge if the cables are working like they should making them the problem rather than the valves.
Getting that info would then drive the decision of going for replacement of one or more valves or possibly working to get some debris cleared to let a good valve close fully!
I would want to get more info as it may be as simple as part of a plastic shipping strap has finally drifted out of the tank to get hung in the valve. If you can find it and reach it, possible a stiff wire can flip it out???
Lots of hope and prayers with that but I like to dream first, work second!!
Just the way I would go!
And pictures are always nice for getting a better idea of what you may have so they would be welcome to most folks.