This is a sore topic for me…
On a doublewide I have replaced the underbelly with closed cell spray foam. Looking back I would not do it again because: 1) it was too expensive at $1.50 per foot (with labor); 2) southeast Texas only gets a hard freeze once per decade and the plumbing was replaced with PEX; 3) you find all sorts of nastiness under there which is a health risk (wear a good respirator at all times if doing it yourself); 4) the windows and other areas on the home also had insulation issues, unless you plan to replace those too there is not a lot of thermal benefit.
I too spent a lot of time evaluating all the approaches to replace one. They are all very labor intensive and most of the time do not provide a good seal to the home. Batts will get animals up in them and degrade over time. Replacing with a thick plastic as suggested is nearly impossible to navigate around all the piers and plumbing. Foam board would require all sorts of intricate cutting. There really is no good way to do it unless you jack the home up a few feet.
A singlewide I have with similar energy issues I replaced the package unit with newer window units. They will run all the time, but for about 50% of the price of an old built in unit.
I would not replace an underbelly again. There may be other reasons if you’re in a much colder climate. Good luck!