I had them put on the rear and the front. My front location for the jacks wasn't the best and we don't use them that much, but the rears get used every time.
The fronts:
This isn't the best spot for the front jacks because they are on the frame
in front of the corrugated collapsible section designed to fold up in a crash. Its a bad spot not because they interfere with the crash worthiness (probably do only a little) but because the corrugated section can bend. Thus putting the jacks down and tightening them up just lifts up the front section of the RV and doesn't really stabilize it that much. I'll likely move them some day but haven't figured out where.
Now the rears, on the other hand, have worked great:
They are welded to the frame at a slight angle so that the bolt end sticks out right next to the hitch. The slight angle works to help left/right stabilization because the jacks aren't parallel to each other.
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