Any input you get will be, for the most part, "wild guesses and speculation", so here's mine. When you were at the campground, you used your toilet and partially filled the black tank with solids. There likely wasn't a lot of water also added to the tank and when you pulled the plug to empty the black tank, those "solids" moved toward the drain, effectively plugging it, so only a "slight trickle" was discharged. When you towed the trailer home, the motion from towing broke up those solids so that once home, they flowed more freely with the liquid that was in the tank and there wasn't a "poo plug" to partially block the exit at the valve.
Typically, only empty your black tank when it is completely (or almost completely) full. I always fill the black tank by flushing the toilet until it "gurgles" before I pull the valve handle to empty the black tank.
My guess is that you weren't level (not that being level is a requirement to empty the tank) but that you were "just enough off level" for the solids to migrate to the valve, covering/plugging the exit, so that when you did pull the handle to open the valve, there was enough "stuff in the way" that the volume inside the tank couldn't push the "plug" out of the way.
I may be way off on my guess, but if my experience, being "slightly off level" has never hindered tank flow. It "may" hinder complete empting of the tank if the valve is above the tank bottom, but everything in the tank that's higher than the valve will (or should) freely flow out of the tank when you open the valve. The only "problem" with being off level is complete drainage, not initiating flow. So, I don't think being off level caused your issue. Probably the solids in the tank coupled with not enough liquid was the problem and that was "solved" with the agitation of towing home when it broke up the solids into smaller pieces.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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