Can you take just a lien on the land and not the dwelling (i.e. dwelling is a mobile home)? I'm thinking in the case of the mobile home, that should be fine but wanted to check. Does it matter what state you are in?

If the mobile home is not considered part of the real property, then yes it is possible. Whether or not the mobile home is part of the real property would be governed under State law.

I have seen that exact situation in CO, but it appears to be isolated to certain counties.