Is idling the engine for 8-12 hours straight bad, on my 2015 Toyotoa Corolla LE ?

not at all. Cars are designed to run as long as there is gas in tank.

this will put a strain on the alternator somewhat , depending the currant draw

Your 'maintenance required' light may come on sooner than expected, well ahead of mileage for oil-change. The ECU counts running time and engine revolutions, as well as mileage. But those guys are right, you can idle it from now until doomsday with no harm. Just be sure to take a drive down the highway for 30-40 miles at least once (preferably twice) a week to keep EGR and everything else in good working order. If this is going to be a long-term thing, consider a 120 or 140 amp alternator, like 7jimmy7 says...will not tax it And I don't know how to do it with EPC that it has, but see if you can find a way to idle it at maybe 1200 -1400 rpm. Now, all that being said, you do know you can buy a 120V AC generator, self contained, for $125...Check Amazon.

just remember if something goes wrong for expl. heat increases you wont be able to see it