I installed a Coleman Mach 8, 15K btu, on my 58 Overlander. I wanted the lower profile. Here in the NW it appears to be sufficient, although we had some days last summer close to 100 and a single unit was borderline. The issue (IMO) isn't the unit as much as the inability to insulate in the 1/5" ceiling/walls very well and the radiant absorption of the aluminum. I plan on painting the roof white this spring to see if that helps much. Last September I added 4 PV panels at the rear so we'll see what affect that has also.
Being in the Midwest and dealing with the high humidity adds another issue for you. Hopefully, someone else can provide additional insight for you.
You can run a single a/c unit with 30 amp shore power, but the start-up surge may be an issue if you have any other large loads operating (mine pulled 22 amps briefly). I added the micro-air easy start and have no problems using 30 amp shore power (now it pulls less than 15 amps on start up). Follow the a/c manufacturers recommendation for wire size as a minimum. I tend to go one size larger to compensate any voltage loss (probably overkill). I also follow Lewster's recommendations and use marine grade wire.
I added the heat coil to mine. Figured it was easier to install it while the unit was on the ground. Is it practical? I don't think it can warm the entire trailer. It could possibly take the chill off the kitchen area, but that's about it. I installed a suburban furnace with duct work to replace the panel heater by the door. I also ordered my unit with the built in condensate pump.
In case you currently don't have any additional support around the a/c opening I would recommend doing it. I took 1.5" square aluminum tubing and created a square support in the roof, sized to the required opening for the a/c unit. It is tied to the ribs in front and behind the opening. It flattens the outer skin a little at the opening, but I think I get a better sealing surface that way.
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