I worked on P30 motor homes at a Chevy dealer for about 7 years back in the '80's, and I did it from the top, though the dog house, and from the bottom(yes, it's a lot of getting up and down).
There's a curved cross member in the frame, right in front of the harmonic balancer. It's held in with 4 bolts and it's got a brake line bolted to it. Removing that helps a lot.
Removing the heat shield on the drivers side is more trouble than it's worth for belts, because the steering gear and brake master cylinder are still in the way. Most '86's won't have a factory installed shield on the passenger side.
Never an easy job, your arms will bleed, and your back will hurt from getting up and down to move between the inside and underneath. If you're not careful, you'll get grease on the furniture or carpet.
Good luck, and have fun.

Oh, I always used a grease pencil to mark the belts. One mark on the first one off, two on the second and so on.