Thanks for the questions Suz...
The jacks have been welded on. I had it done at the same time I was having the frame repaired/reinforced.
After we arrived in Seattle on our first trip - we noticed a crack in the frame where it bends inward to form the toungue. (It's a known weak spot in Bolers. I hesitate to call it a 'flaw'. If something lasts for 30 years without a problem - I call it a pretty decent design.)
Anyway, we took it into a welding shop to repair and reinforce the frame at that location (and the same on the other side). The repair is actually visable in that shot.
Anyway - we brought in the four jacks to install at the same time. To attach them to the frame, they first welded the jacks to a U-shaped piece of steel (at the bottom of the U). The U then slipped up around the frame and was welded into place. In other words - the U helps to act to reinforce the frame in that region too.
I think it would be a mistake to try to bolt something to the frame in that location. The is already a high-stress location (it's roughly midway between the axle
and the hitch) AND there's a bend nearby too. Weakening the frame further by drilling a hole through it wouldn't be that advisable... IMHO of course.