Hope you find the manual, but I think yours has been modified. Not sure that will affect its operation, but there may not be instructions for the mods.
Porta pottis typically have a chemical flushed, or poured into the tank. It is available at most marine or RV or camping stores and I'm sure the correct quantity is on the bottle. Its been a long time since I've used it. To use the bowl, you may want to first do a very small pump
to lubricate the bowl. That lever is likely your pump
, but some have an accordion style pump
. Once you've done your business, you pump
again until the bowl is clear of what you deposited. If you don't hear from them, it should be an easy experiment to figure out.
As for the pumpout, if it was added by the PO, there is something that concerns me. To use it, you would just go to a pump out station, press the vacuum in the deck fitting and pump it out. That's all that should be required. The vent will allow air to replace the volume. The concern is that top fitted pump out intakes never get all the product out of any tank. They need some space at the bottom of the tube and once they've drawn as much as they can, some will drain back down the pump out tube into the tank. Think of a straw. I'm not sure porta potti material is designed to have waste sit in them essentially all the time and once odor permeates plastic, you can never get it out.
Just some thoughts. Hope it works out. You have nothing to lose to try it.