Here's my standard reply when somebody asks about adding onto, modifying or building a house over a mobile home: Don't do it.
You'd be a lot better off to build what you want from scratch rather than trying to modify a double wide. The frame of them is the absolute bare minimum adequate to support what the manufacturer puts on top the frame, and is NOT intended to handle whatever else you want to pile on top.
Every time I see a 'build on' to a manufactured home, it's almost always done badly, by folks who have a dream, but no real understanding of construction.....and the result is almost always a mess.
That said, every single one that has ever asked me has proceeded to ignore the advice, and done it anyway....so feel free to join them.
Neighbor down from me asked TWICE....once when he was making a "double wide" out of a single wide...( I said don't do it ) and then again, when he asked about digging a basement under the mess....which was the frame of the old trailer, still on blocks, and the addition, which wasn't on ANYTHING but 4x4 posts on scrap blocks of 2x8 for "footers"....looked like a forest of them under the addition.
Now he has about 80-90k in the thing, and God only knows HOW much labor in it and still looks like what it is.....a single wide trailer converted to a house with a leaky basement....and he wonders WHY he can't sell it.
Another neighbor added a 12x20 addition onto the side of a 14x70 single wide. He put block piers 16x16" filled with concrete ( 2-8" block cross layers ) all down the out side, the inside, and another set in the middle of the addition....because he didn't know how to do a continuous foundation, which would have used less block, concrete AND enclosed the addition ( which he later "under pinned" ). Then proceeded to use finger jointed 2x6's for floor joists, because "they were the same size as the joists on the trailer AND a whole lot cheaper than those regular ones"......ahahahaaa....and so on and so on.....a mess.....