Sewer Drain

I think also will loosen the screws next time as well.

Apply directly to push/pull bar.

Where are you applying the lubricant? WD-40 on the push/pull bar makes mine work better immediately. There is arguably better grease available.

Possibly something stuck in the slide valve seal. If your valves are cable operated then you should to check the cable for kinks or corrosion. If it's a real problem I think the valves are less than $20 so it might be easiest to just swap it out. Assuming your tank is empty.

Also, you might want to check the screws in each corner of the valve. If they are too tight they tend to squeeze the blade and make operation difficult.

Have you tried aggressively cleaning the tank (with something like this: http://www.amazon.com/gp/product/B000BUQOCC?psc=1&redirect=true&ref_=oh_aui_detailpage_o09_s00)? Depending on the age of your trailer, there could be manufacturing remnants stuck in the valve, or , "tank stuff" from use.

I have had that problem before too. I drilled a hole and put an automotive grease zert in the flat part of the valve where the rod goes back and forth. gave it lots of grease and the problem went away.

I have added some valve lubricant, slippery stuff to the tanks, banana oil perhaps?
bumpy