My 89 345 has the Coleman Super Mach series model 6798 Air conditioner. I believe they are original. If you have the same or similar, I may be able to help. You have to remove the lower cover and control assembly. The inside tube location is in the divider section where hot air goes uo in and cold air comes out of the vent area. It looks like a square that's divided into 2 rectangles with a 2" divider. This divider is a gray plastic on mine. If you're looking to clear out a plugged drain, go at it from above. I took the plastic cover off from the roof and was able to clean leaves and other debris from the divider drain parts with my hand.
The lower end of the drain is above the tag axle on the driver side. I blow the line out at the beginning of every season. Do this by putting the air line on the lower end near the tag axle.
"Sometimes I wonder if the world is being run by smart people who are putting us on, or by imbeciles who really mean it." Mark Twain
AIR 22749 WBCCI 2349 NOVA TAC TN-6
1989 345 LE Classic Motorhome