As far as I know, Lexi's do not come standard with an inverter. The converter on our Lexi 283ts is under the rear bed. But no inverter came with ours.
It might be an option, or added by the previous owners.
If there is one, it's usually mounted close to the house batteries. On a 283ts if one was added, it might be under the fridge. Thats the only place near the battery that has space enough for one. If not, look for any added wires to the battery and try and trace them.
Also, if you bought it used from a dealer, salesman might have confused an inverter with a converter, telling you it had an inverter when he ment a converter, which is standard equipment.