How to keep critters from getting under a mobile home?

is there a way to put some window screen behind the skirting? or replace the skirting with kind that has less places for a varmit to get into-- a more solid one.

I think a more solid skirting would do it .

Personally, I would back it with chicken wire.

"Hardware cloth" screening should keep out critters. Make it sufficiently long to double at top and bottom, and secure it, tightly. Throw cotton balls saturated with peppermint oil, clove oil, etc. under your home.

mice and small critters would go right thru chicken wire. I still think the best is to replace what you have with the solid panels that are made mobile homes. they go up and down and can be cut to the length needed, maybe even into the ground a ways to keep them from digging under.

Almost all animals are repelled by oils like citronella, peppermint, eucalyptus. You can plant citronella come spring around. It grows big and repels mosquitoes also. Moth balls can be thrown under the home but I don't prefer the smell. Citronella smells nice, so does peppermint.

Get a cat

Not sure if this is feasible, but measure that slatted bottom section to apply garden weeding fabric to the outside(leaving extra length to put under dirt too). Tack it to the vinyl with something that can line up with the current fasteners (case access is needed later)? Then put pavers, hardscaping over the garden fabric section?

Definitely agree with planting rodent deterrent around perimeter too.