Have been using X-chocks for over 5 years. Have NEVER ever once found them to get loose after they've cooled down and that's in temps from around freezing up to around 110F. I tighten them up nice and snug. I must be doing something wrong (sigh)??
I can't see how the Fastway chocks would be superior to X-chocks (providing they're tightened) to prevent motion inside. Doesn't make sense from an engineering perspective. I think the reason that BAL has the statement about not using them as a parking brake is because how do you put into words how to tighten the X-chocks enough for the average Joe camper. Can't expect owners to use a torque wrench.
If we were camping on a steep site, which we never have been and likely never will be, I'd want something better than X-chocks or the Fastway chocks.
My method of reducing motion inside starts with the strong HD frame we have, then X-chocks, BAL lock-arm struts at the rear and Torklift Glowsteps. Very little movement inside and anyone is welcome to come to a CG we're at and try it. The electric "stabilizer" jacks are next to useless for stabilizing. The tongue jack reduces movement, although may not be much but you can add re-enforcement to it on the underside of the coupler/A-frame and use a larger pad. A pair of jacks ahead of the axles did very little to help and don't bother now after a few uses of them. One of our top mods/upgrades has been the "stabilizer" stabilizing struts.