Re your toilet problem: Not sure what's causing it, but I have a strong suspicion it's caused by a blocked holding tank
All tank vents have two main functions: to provide an escape for air displaced by incoming contents and provide a source of air to replace contents as they're pulled out. When air displaced by incoming waste cannot escape out the vent the tank becomes pressurized, creating increasing back pressure that prevents the toilet from flushing
Without a source of air to replace contents as they're being sucked out of the tank, the pump will pull a vacuum that'll prevent it from pulling out more than a gallon or two. The pumpout couldn't pull in any air via the vent so it got air wherever it could...in this case, through the toilet.
The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning
out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck
pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...reconnect the vent line.
As for your toilet....The Aqua T is made by Johnson Pump, a Swedish company owned by SPX...their products are pretty decent quality. They have a US operation and offer support there https://www.spxflow.com/johnson-pump-marine/support/
I was able to find the instruction manual for it online. https://www.spxflow.com/assets/pdf/J...-412-1_R06.pdf
It's in about 8 different languages, pages 9-14 are the English
version. You should be able to print just those pages.