Around here - lots of folks use bales of straw. Cheap, good insulation value, simple installation, easy to repair. NOT allowed in most mobile home parks. Looks rather "honkey" and does increase the concern regarding fires.
I'm on a slab and built a knee wall with conventional techniques and then plywood ....... and finally vinyl siding from top to bottom.
No insulation but the flooring insulation is so inadequate in mobile homes that the temperature below stays around 40 degrees even when at zero outside. ( remote thermometers are a good investment )
After 40 plus years my best advice ...... DO NOT put your money in a mobile home. They are a very bad investment and generally a pain to maintain.
If you are in a cold area insulate the skirting then
cover the sides and floor with heavy plastic and your
floors will be nice and warm when it gets cold and no moisture wll bother the bottom of your mobile home.
Also you have a metal roof to keep the heat out/in get this silver stuff for mobile home roofs or if you have a reg house roof make sure you have plenty of insulation to save money.