Installing thermopane vinyl windows in an old tin sided mobile home

First, this metal sided mobilehome is a 1973 Concord, but still in very good shape (only 1 ceiling stain from an old leak and no mold spores during testing)

The company that made the windows botched on the sizes (made them for rough opening instead of exact opening) so there is a gap around them (probably 1"x1" or 2"x1" will fur the gap) I'm adding siding with insulation afterwards so I'm not so worried about the looks at the moment (just not freezing my butt off this winter in Seattle!)
So what I am doing (and have done on the smaller windows I put in already) is removing the original window, then I cut the exterior sheet metal along the edge of the top of the opening (they wrapped the metal inside around the opening when they built these back in the day) and notch the corners of the fin on top to the edge of the frame. I'm thinking about adding that sill wrap tape for the next ones, then tucking the top window fin between the mobile's sheet metal exterior and the inner frame (which is probably 2"x3" or possibly 2" by almost 4" stud/sill) Then another piece of the sill tape on the outside to keep water/drafts from getting in. (it will all be hidden under the siding I plan to put on)
I got the windows made for 2"x6" walls with centered fins so they stick out far enough to be even with the siding that's going in.

That's as far as I've got so far.

Just before I installed those windows I found out about the sealing tape needed and that cut the draft down a LOT. Our thermostat is the same as last year (before the windows) and outside temps were about the same, yet it cost $100 more to heat during this time? That doesn't make any sense at all!

Without a picture it's had to say what they did.
To late now but you should have just refused to let them install the windows.
The cost of fuel has gone up, that may be the reason for the higher heating cost.
It would be interesting to see how you plan on adding the siding. Insulated siding in most cases uses 1" J molding so it's going to be a challange to trim it out around the windows and doors. It also cost at least twice as much as reguler siding.

I put the windows in myself. Insulated siding will more than double my walls R-value (twice nothing is still nothing though) The "thickness" of my walls (3" plus the tin siding and paneling on the interior) means the 4" thick windows stick out on the outside (instead of on the inside where someone could hurt themselves by walking into the sharp edge) so the siding will have something to go up against.

I'd like to see an outside picture of that one.
Are there's new constrution or replacement windows?

replacement windows. Originals were single pane with a storm window on the inside.

Sorry I see no way that install is going to work out in your favor.
Reasons being your trying to seal a flat surface to one with ribs.
Wood trim will end up rotting out from water laying on top of it.
Just look at any wooden shed door with exposed cross bucks on the outside of the door. The pieces rot as well as anything behind it.

I would have set the nailing fins in a bed of silicone, installed window tape cut to the width needed to cover the nialing fin and about 1" of the wall, then use vinyl trim also set in silicone with any horizontal pieces ripped so there's an angle at the top so water will run off.