OK, 10 years late, but here’s the answer:
The joke is recorded in Gershon Legman’s Rationale of the Dirty Joke, p. 59, which quotes sources from 1907, 1933, and 1934.
Apparently the joke was current in the 30s and 40s and was even the reason for the ban on the phrases “Hold your hat” and “Hold your hats” from movies during the period by the Motion Picture Production Code.
The mountaineer and his wife and their three little boys all sleep in the same bed together, the boys wearing their coon-skin hats to keep warm. During the parents’ intercourse, the bed collapses several times, the boys’ hats flying in all directions. The parents wait till the children are asleep and try again. Just as their orgasm is approaching a tiny voice shouts, “Hold onto your hats, boys! Here we go again!”