I have read that 'pikers' are/were Missourians. It had to do with Pike County, wagon trains, westward expansion (maybe gold rush?) type stuff. "Sweet Betsy from Pike" and all that.
If I understand correctly, the negative way of using the word has something to do with being a layabout, avoiding responsibility, not doing your part (this is not a proper definition..just kind of how I understand the word)

Any connection with the modern English usage of Pikeys as a description for 'travelling folk', the etymology given in Urban Dictionary as those who encamped around the 'turnpike'?

Having said 'moden English' usage, I note the first citation above dates it back to the 19th century!

Nigel