And as you can see here, it shows up in print as early as 1918, in some sort of newspaper or magazine known as the Peregrine Observer:
But that place no longer exists (that I could find) - although there is a "Dismal Swamp", Arkansas. Could it be that Dismal Seepage came from Lum and Abner rather than Paul Rhymer? There's no way of knowing but we know that Chester and Chet were friends of Rhymer (Lum and Abner even have a section of the main website devoted to them).
I talked to Donnie Pitchford yesterday (he's the big muckity-muck over at the National Lum and Abner Society); he says Clarence Hartzell was all about some Dismal Seepage when he talked to him. Neither Pitchford or his cohorts know anything about Dismal Seepage in Arkansas. From what I gathered, they tend to think it's a Paul Rhymer thing...
So, who knows?