I would gladly buy a steel or bronze acme nut for this slide mechanism.
A shear pin is an inexpensive, easily changed part that is sacrificed to prevent damage to more expensive parts. The plastic nut that costs $1500 or more to replace makes a pretty miserable shear pin.
Note to Winnie: replacing the nut requires removal of the motor, which needs only 2-3" of clearance. That's not to hard to engineer when designing a RV.
Note: to Kwickee/Power Gear: If the nuts two dowel pins were screwed into the nut (and then secured by lock wire or a clip) the motor (or complete assembly) wouldn't have to be removed. Acme nut replacement could then be accomplished in less than an hour.