I did some more testing and I got my normal 110V window a/c unit, which was installed in the rear window opening of my van for use while parked, to work long term while driving!

I found the problem was the ground wasn't good enough and the inverter will shut down due to low voltage. I was fooled because under heavy loads, it doesn't give a warning tone but just dies. It also appears to have a latching mode, as long as the inverter switch stays on it will remain off so it doesn't keep coming on and turning off. Simply turning it off and back on won't work either, it has to stay off for a minute or something like that. That's a Solar power inverter, which is an expensive but good one. Relatively small and very low idle current, too.

Needs a big alternator like the Motorcraft 3G, preferrably the 160 amp version rather than the 130. It needs a battery isolator capable of handling the 160 amps too. The a/c seems to take 70-90 amps out of that, it might be varying a lot though, I'm not totally sure. I thought I saw 45 amps on another test. Might vary based on its evaporator and condensor temps. I need to pay closer attention to this issue.

It can't use the stock ground either. There's a stud on back of the 3G that you can mount a 4ga wire to that can route straight to the battery. Awesome.

My a/c is unfortunately an electronic one that has to have the "ON" button pressed once it's powered up. Being in the rear this is an issue. I did finally find the remote for it though, and can now hit it from the front as long as the view is unobstructed by cargo.

It definitely cools better with the rear a/c. The effect is slow, normally you get blown directly with cold air from the normal a/c but the van as a whole doesn't cool down. The rear a/c doesn't blow on you directly, if you leave the front a/c off it takes like 10 min of driving before you notice it cooling down but it actually cools the whole volume of air in there. The front a/c's discharge temp drops down a lot when it's not sucking in hot air too.

It's nice but I worry about running that high of a current through the alternator when it's like 100F+ here in Texas, which is exactly the conditions I'd most need to use it in. A blown high performance alternator in the desert would indeed suck!

 

Thanks for sharing the tips/info.
I had a '93 E250 standard length w/AC. Thought the AC sucked 'cause it took so long to cool the van. One day in, the middle of July, in Florida, I had the van literally stuffed with carpet. Turned the AC on, three minutes later had to turn it down. The realization dawned, gotta cool the whole van before you feel it up front. Put up a set of those metal seperators behind the seats w/homemade plexiglass sliding door in the middle. AC was always cold after that.
Munrow

 

More testing shows the current is indeed related to the temp differential.
Right after starting it takes only 45 amps, but as it runs it slowly creeps up to like 90 amps. Maybe more. And that was tested under some fairly cool conditions.

I'm a bit concerned as to what happens when this thing runs in 100F+ heat. The current draw might go way over 100 amps. That could explain some of the problems I saw before.