Tecmas are excellent toilets! However, although "pressure relief" valves are more common on tanks installed with VF toilets, they can be found on other tanks too. It's intended to prevent an especially strong pumpout from imploding the tank by opening if the vent is blocked (as can happen if a filter gets wet) when the tank is being pumped or dumped.
But I don't think it's causing your problem...I suspect there's a blockage in the vent fitting on the tank that's a result of overfilling the tank and allowing waste to build up in that connection. Happens a lot on sailboats 'cuz heeling causes tank contents to spill into the vent...not so common on power boats unless the tank is allowed to stay very full, maybe over-full. However, the vent thru-hull is the most common location for a vent blockage, so unless you've already successfully blown air through it with -0- resistance, I'd scrape it out too...use a screwdriver blade, ice pick--whatever works--on both ends of the vent line--the vent fitting on the tank and that end of the vent line.
Fwiw, all boat builders use the same type of thru-hulls on all tank vents--water, fuel and waste. They're designed to keep sea water out of the fuel and water supply, but it hardly matters if a little sea water gets into a holding tank..and they also restrict the exchange of air with the gasses in the tank. So I'd replace that vent thru-hull with an open bulkhead or "mushroom" thru-hull that you can stick hose nozzle against to back flush the vent line every time you pumpout or wash the boat--preventing future blockages. You'll find details and photo of the type of thru-hull on pages 41-43 in my book.
If you overfilled the tank, it sounds like you don't have a tank level indicator (or one that works). Check out SCAD system Scad Tank Monitors
Easy to install because the sender goes on the outside of the tank so it can never become clogged. The SOLO single tank monitor is very reasonably priced and even available from Amazon