Don't think any of the generators I know of use Diesel as they are fed off the onboard ATV fuel system which is gas.
Number of AC units will depend on the size of the trailer as will the size of the generator.
There are some reasonably priced new trailers out there in your price range if you'll take something a bit smaller. I think you will also find that the max combined weight of your truck and a bigger trailer will easily exceed the recommended max weight. This has to do with the axles, tires and brakes on the pickup I think you'll find that for your truck, it's going to be about 16,000 lbs. One of the other things is the overall size of the trailer including width and height. A lot of the newer trailers are 8 1/2' wide and on the order of 13' tall and will push that 18,000 lb, loaded weight you mention. You make it 40' long and it'll be a real pain to get around especially in town. Those size trailers will need three axles.
In my experience, weight is the real killer. Worse mileage, every hill is a "mountain" and they are hard as hell on tires as those get the tread scrubbed off every time you make a turn.
In view of the fact that I originally anticipated taking mine off road, size and weight were very important considerations. Height? didn't want it getting all scraped up by trees along dirt roads. So 13' was out of the question. The trailer I got is 11' tall and pretty easy to get under all the tree limbs. Width? same issue, I didn't want anything wider than 8'. It's 34' long and I originally figured on hauling ATV's. Now it hauls model airplanes so I do not know how big the garage is and it is separated from the living quarters by a wall which I had them put in a sliding glass door rather than the standard type wood doors. Also makes it easier to store things as you do not need to keep space clear for the regular type door to open.
I think that a divider is really important because things are going to get spilled in the "garage" and the living area doesn't need to be stunk up by the spills. My "garage" floor is aluminum diamond plate. Easy to clean and good traction for ATV's as well as people with wet or muddy feet. If I have a complaint or something I'd like to change, it'd have bigger water tanks on it but there's only so much room and the 25 gal. ATV/Generator fuel tank takes up some of that area. The generator on mine is a 4000 W Onan and it feeds the singe AC unit, TV, Satellite antenna and such. As I mentioned, I'd have liked two AC units and a bigger generator but nothing larger was available for that size trailer. It has one slide out for the kitchen/dining area.
Now here's possibly the best part; it only weighs 11,000 lbs. fully loaded and my 1 Ton Diesel Dually handles it with ease. It's made by KZ and the model is called a Sportster.
