Thorin and the Dwarves once coal diggers?

[Kuruharan]

...unfortunately, I don't have much time to devote to discussion at present.

Did Dwarves after the fall of Erebor resort to digging coal and doing other dirty unwanted jobs like shoveling dung etc?

I doubt dung but I think it likely that in the early phase of their exile the dwarves formerly of Erebor probably did dig for coal.

Where were they digging coal?

Dunland is one place specifically mentioned.

Did they work with other dwarves?

Probably not until they got to the Blue Mountains.

Was the payment decent or not?

It probably wasn't particularly.

However, I think once the exiles of Erebor settled in the Blue Mountains they were able to mine for other things than coal. The Hobbit indicates the exiles of Erebor were prospering again to a degree.

Gandalf was probably to some degree speaking with a bit of classic Gandalf sharpness when he made the coal reference.

For further reference, here is a thread that talks about this specific period of dwarf history in some detail.