If the fuses are good, check to see that you didn't loosen the plug at the wiring harness. That's right behind the outlet. I really think that's the most likely scenario.
As for using the outlet for a battery tender, it is just fine for that purpose. Optimate badges one of their units with the Triumph marque, and Triumph sells them. You can buy a plug from Triumph, with a Triumph part number, that will plug right into the powerlet plug to connect the Optimate and tend the battery.
I have both the Optimate and a Battery Tender. I have a pig tail on the battery of the R3T, and I have the powerlet outlet there on the left side. I generally use the Optimate unit through the powerlet plug on the R3T, and I generally use the Battery Tender on the supermoto, lawnmower, etc.