Holding tank lids stuck: how to open?

[Krumelur]

I would like to clean (flush) my holding tanks but I cannot get the lids to turn. I don't think anyone has ever used them to pump stuff out.
The tool to open them just bends. What would you suggest? Heat?

[Odysseus]

Short length of wood with 2 screws strategical placed to fit, then lever it open.

[tiger79]

I had to destroy my tank access port to remove it.  SVB sell replacements.  I fitted the new one with lots of silicone grease so hopefully it'll unscrew next time.

[sunshine]

Pair of pliers in the fully open position. Insert the handles into the two slots on the tank cover. Use a spanner (I used a shifter) on the pivot point of the pliers and turn. If your pliers have plastic handles, so much the better.

[Ailatan]

I used this tool