Most 10-yr old motorhomes in the Sightseer price range did not come standard with an inverter. OR if they did it was a tiny one to power TV and satellite gear.
Are you sure you have one in your coach?
I checked the 2010 Sightseer brochure and a 300w inverter was Optional on your 33C motorhome.
That sized inverter would be to power entertainment (TV, etc) only. And those are usually located near the overhead compartment tech cabinet - likely at the front of the coach or wherever you have TV and Video entertainment equipment.
According to your Operator's Manual in Section 8-4 it's located as follows:
The inverter is typically located in the video center cabinet or on the lower face of the galley cabinet.