The amount of return air will be determined by the free air space at the return grille and the filter.
Your home is the return air duct/pathway.
Not ALL conventional homes have a return duct and use the same pathway as a MH.
It would not be designed for a MH if it could not be used.
Most HVAC contractors either do not understand MH's or just do not care to work on them.
Same can be said of electricians and plumbers.
THAT has forced MH owners, humans who need heat and air like everyone else, to do it themselves or do without.
That is the very reason I went to almost all MH work 5 years ago and a little over 2.5 years ago joined Mark as an employee through this website.
That's why when I came here over 8 years ago, I never left.
Nobody should ever do without heat or air anywhere, especially in this country, regardless if they are in a MH, igloo or cardboard box.
If they are forced to DIY, we will do everything in our ability to make sure it is DIY'ed right and safely.
In your situation, the gas pack would be a good option.
It has all heat and a/c in the one outside unit and only needs wiring and two ducts ran, return and supply.
Supply connects to main duct trunkline and the return is connected to a return air grille set up in home.
That would need to be cut out in floor most often, but depends on current home and unit layout.
Sorry for rambling a bit, but this issue is a sore spot for me and has been for 28+ years.
Take care and best wishes,
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?