With a good jack or lift table and a few mechanically inclined friends you can do the trike conversion yourself if you really wanted.
Depending on what bike that you are converting I think your MPG's will decrease a bit but in my own opinion and experience I'd guess that you would lose 20-25%.

Not quite "apples-to-oranges" here but I have a '13 CVO Ultra S/E with the T/C 110 engine and Stage I kit and I get 38-40 MPS cruising at 3000 RPM's.
On my '18 Tri-Glide with the M-8 107 engine and Stage I kit I get approximately 32+/- MPG's. (ranging from 25 to 37 MPG's depending on road incline, wind and speed).
My lowest MPG's was on an increasing elevation trip with a heavy head-wind and my best was on a flat two lane road running between 60-65 MPH's.

In round numbers the Ultra weighs 900lbs and the Tri-Glide weights 1200lbs which (again) in round numbers is about a increase of weight that you are pulling.

Make no mistake, the ONLY thing that you gain in going from 2 to 3 wheels is stability and everything else is give-a-way.

 

Stability is my main concern and I'll have to deal with the give-a-ways cause I ain't gonna give up riding. I'm sure I need to understand how it will handle curves and master it.

 

I was just told about a company called Tow-Pac Inc https://towpacusa.com. They seem to have a number of options and their prices ain't that far from one of the standard Voyager Kits. Anyone else ever hear about them?