Welcome to the forums!
We bought ours from Family RV in Florida. They ship from a warehouse here in Ontario, CA. It was a better price than Palomino's. We got the Carrier 15k btu, but for your 20' Argosy the 13,5k should be enough I would guess. Or if you like it really cool then go for the big one
. They come with or without a heat strip. We already have a heater so we got it without. We chose the regular, not low profile to keep costs down. I know others have bought from Palomino's, they are a trusted seller by many. There are many others, you just have to shop around. Do an online search. Here are some links:
NEW CARRIER 13.5btu RV AC ROOF AIR CONDITIONER SYSTEM:eBay Motors (item 140341742556 end time Sep-23-09 11:28:59 PDT)
COLAW RV PARTS & SALVAGE | RV Rebuilders, Wrecked, Refurbished RVs [Recreational Vehicles]
I hope this helps.