Hi. On my Freightliner I went with the popular add a platform to the back.
There are several reasons I decided to go this direction.
I plan on installing, and already bought, two mini-splits.
After the 4 foot add on is complete including lights I plan on mounting the outside units in parallel, towards the outsides with the air blowing out from the middle.
Another reason I like this is I hope to put a roof over this addition and make it into a little porch like I've seen others do. With a railing around the outside the units will have some protection as well as conceal them just a bit.
Someone pointed out the very end of my new addition might scrape the road when going up a steep place, like into an uphill driveway. I'm planning on addressing that with the wheels used to stop a fifth wheel from scraping.
A significant concern, and one I've spent minimal time on, is finding out where to get the plastic air conditioning tubing like the bus uses for the built in air conditioners instead of the copper tubing that came with the units.
After adding the handicap bathroom and shower I plan on the maiden voyage. This first run probably won't be for a too long time.