Look into "ductless air conditioning" also called "mini split air conditioning" it's the way to go especially since you have two distinct areas to cool. They also have heat pump models.
They can be a bit pricey, but you can pretty much put the condenser unit anywhere and the inside unites are small and good looking.
I was going to go that route, but I got a bus with factory air, which is basically a mini split without the nice features and it runs off of the bus engine, I'll have to do something about that.
There is some discussion both good and bad here
13,000 btu unit for around $350
I don't think this would be near big enough. I have a 9,000 in the cab of a semi, if I'm sitting in the sun, it's pretty much sit right in front of it to stay 'kinda cool. I think the biggest roof air you can get is 15,000 and the large bus/motorhomes always have at least two or three, and I saw one with 5 yesterday.
take a window unit and build an insulated air tight box around it leaving space for the air it needs through the back (and side?), vent that to the outside of the bus and leave the front of the a/c popping out at the foot of the bed and shooting cold air down the hall
you have to have some kind of drain for water somewhere.
You will have to do your own research and conclusions. You have a unique situation, not sure it can be resolved cheaply or easily.
Will a 466 and Alison fit where that Bristol is?