I have a 51 Silver Streak Clipper that has no AC. I asked the same question on Tom Pattersons site and got a very helpful answer from Eddie Huffstetter.
I am considering doing this if I can find a place for the unit to go.
Here is Eddies way of doing this...
The whole issue is vintage RV's didn't have AC. Roof air conditioners present concerns or issues of ugly cosmetics, roof structural design support, and heavy AC's make all coaches top heavy. Worse, geographical areas that might not present a need for AC, RV's often have atrocious ugly swamp coolers on their roofs which are even worse. Generally speaking I do not like roof AC's and especially on any vintage RV. So I do not use nor mount roof AC's on vintage RV's ever.
I like and install belly air conditioning. Today's expensive coaches often don't use roof AC's either. They use belly AC's. Even expensive Roof-Air coaches, use ducted systems. A belly AC system allows you to keep a coach period correct yet have modern central air conditioning. It is really not hard to do.
Basically, any window unit or portable AC unit will do. A window unit can be substantially cheaper than any roof ac. Any location in your coach will also do such as the floor of a closet, under a cabinet, most any area. I like closets. You get to keep the upper part of the closet or cabinet since not a lot of room is needed.
The belly air needs to:
Exhaust hot air out of the coach, get operational intake air from the outside, put cooled air into the coach, and it needs a way to get air inside the coach returned to the unit. So you need hot air out, room air returned in, and cooled air distributed throughout the coach. AC's make lots of water. You need a way to get the water out. You need electricity to power the air conditioner. You'll need an outlet. You need a way to control the unit. I make a thermostat controller. It is something I can tell anyone how to make. With the controller, you simply set your wall thermostat just like in your home, for the temperature that you want. With this controller, you just plug the AC into it, then plug my controller into the wall outlet. Therefore you make no alterations to your air conditioner unit. You set your AC unit controls to max, and let the controller with a wall thermostat take over.
Cold Air Distribution:
I make an adapter for the portable AC units for ducting the cold air in the manner that I want. For window units, I make a partition for the cold air. I use Truck type dash flex duct in closet and cabinet corners or make my own ducts. Ducts go up the cabinet wall corners, and continue fore and aft through the upper cabinets. I use muffin fans to boost the air flow thru the duct work, one fore and one aft. You can buy nice register grills for the ends of the cabinets. Those are available in small sizes and often have adjustable louvers too. PVC and all sorts of things make great ducting. Ducting needs to be insulated.
Return Air To the AC:
This is really simple. A simple Filter Receiver-Grill is mounted in the lower closet or cabinet door or the end metal wall, in the case of a Clipper. This gives you a easy-change filer.
If using a portable unit, it already has a drain and hose system. I simply make a small hole in the floor and put that drain thru to the ground. That will need a non-use screen protector you can easily make, but you can buy that. If using a window type unit, you'll make a metal pan for the whole AC to safety sit in and plumb that just like the pan on a water heater, putting the drain once again thru the floor.
Through the floor in all things, means of course the floor AND the belly pan that all Clippers have.
Hot Air Out:
Again, I go thru the floor with this (and the belly skin). The hole(s) are not huge, and easily cut and fit. You have to find the steel floor supports and not make your hole such that it interferes with that support and fitment. Your ultimate exhausting of the hot air discharge ducting must also be screened for varmints and the ultimate no-seeums. I also like to put scuppers such that the exhaust air does not blow directly down on the ground on dusty grounds situations. I never alter the exterior of vintage coaches. Nothing could be worse in my opinion than cutting the outside skin and putting some sort of grill work or vent louver work on the outside of a vintage RV. Portable AC's already have ports for hoses. You may need to fabricate those to redirect as needed. Window units are once again, partitioned cavity.
Operational Intake Air:
This is really simple. I make holes in the floor. Those get varmint and no-seeums screening protection. I use transition ducting from the floor thru the void area to the belly pan. I use scuppers to make sure the hot exhaust air and the cool operational intake air keep separated and cannot intermix.
Window Unit Belly AC units:
In the case of a window unit, I simply put the unit on the floor or built out wheel well or whatever, and I partition that area to separate the return air, cold air, and the hot air. I put a access cover over the area. It is not hard to do. The cold air ducts attach to that cover.
A Clipper is metal clad inside. The metal is easily pulled back for access behind without removal. You just drill out the rivets and expose the area to run 14-3 wiring in the walls. I put two duplex outlets in Tom's 54 Clipper. This provides more than plenty extra ac outlets.
Electrical for Duct Boost Muffin Fans:
Those are cheap and readily available at a lot of places like Northern Tool, Harbor Freight, old Computers, Computer Stores, Big Box Stores, Electrical Parts Outlets, and so on. You need 120VAC Muffin fans. You want quiet ones. The power for those fans are run to the little Thermostat controller I spoke of above. Whenever the AC comes on, so too do the Duct Boost Fans.
I buy the large display Blue Indigo household type. About $40, they are easy to use and all have a time delay built in to protect the AC unit. These thermostats use low voltage (24vac) contactors and so a typical two-wire thermostat wire is all you need to connect it to the controller. That thermostat wire is available cheap at Home Depot.
I'm sure I forgot something. Please forgive typo's and errors.