No suction at all tends to rule out a blocked vent, but I'm not 100% convinced it isn't. At least 90% of vent line blockages occur in two places: the vent thru-hull and at the tank--that end of the hose and the vent fitting on the tank. So you can't be sure your vent is open until you've checked and cleaned out both.
If the problem is a loose waste hose connection, there'd be hissing at that location, plus flushing the toilet would push enough out of a loose connection to at least create some dampness around it. So I'm more inclined to suspect blockage in the discharge line, most likely in the tank at the fitting. Have you had any landlubber guests who might have disregarded your instructions NOT flush wet wipes? If so, that would be the culprit...and one of the few, if not the only, thing that a strong dose of something like Sew Clean can't dissolve. Is there an inspection port in the top of the tank? Sew Clean should dissolve any clog that's actually in the pumpout line.
An off-the-wall thought: is there ANY chance that the pumpout guy "plugged in" to the wrong thru-hull?
--Peggie