Some modify the escape
window (typically in the rear) to handle a window AC. Unless the power center has been replaced and upgraded, a 1300 has 15A 120V capacity at best.
Personally, I'd keep looking if AC is a must. These old Trilliums, which I love, are not cheap. Recent pricing is similar to what a newer Scamp
13 with roof air sells for. You lose the great windows
, but you gain roof AC.
I'd make a list of your requirements, and shop accordingly. Its kind of like buying a car, or a home. I always make a list of requirements and preferences. A preference is something I can live without, but a requirement is a MUST. It's OK to put AC on your MUST list!
Rare, but there were some Trilliums made with roof air. They have a special transition piece on the roof. Finding one is nearly impossible and modifying a unit for roof air would be quite costly.
And realize to have AC, you will need shore power. A generator
"can" work, but many campgrounds have strict limits on generator