That's why they invented folding tongue jacks, right? Then when the trailer falls off the ball, it won't scrape the ground while hanging by the chains! LOL

 

If the chains are sized properly and are the proper length for the truck, yes, they are going to safely cradle the tongue of the trailer. If it's a home-build of questionable integrity, all bets are off.

And yes, there's a good chance it'll drag the jack for a bit if you don't run it all the way up (I always do, so it is barely sticking out - but I have a high approach where I store my trailers so if I DON'T run it all the way up it'll drag anyway). But that drag isn't going to be enough to make your safety chains come undone, and it will be momentary as the shear force is either going to quickly grind away the jack, bend it, or tear it right off.

Regardless, if the chains are still attached to the truck, you do NOT want the breakaway switch engaging your brakes - it applies FULL braking force immediately, locking the tires on the trailer and all bets are off if you can control it or not. Ask any truck driver who had an empty trailer and had to slam on the brakes - locking trailer brakes tend to lead to jackknife situations.

 

If your safety chains are THAT long that your bumper can reach your propane tanks, they're too long. They need to be shortened.

Your safety chains should be sized large enough to support the loaded tongue weight of your trailer, and long enough that they are only an inch or so under your hitch in a relaxed state when crossed under hooked up (they should NEVER be hooked straight to the same side of the truck as the trailer - you want an X shape, or a 'cradle,' to catch it). This provides adequate movement even in the tightest turns, and IF the trailer becomes uncoupled it only drops a little bit before being cradled by the chains and held off the ground. In ideal circumstances (not that being decoupled is ideal anyway), the tongue should only be able to go under the rear of the vehicle a little bit as you slow down, and should still be well off the ground.

If your safety chains aren't able to be hooked up such that they cross under the hitch, they're too short and need to be lengthened.

If your safety chains drag the ground, they're WAY too long - you can either shorten them or cross them multiple times to raise them, the choice is yours though I prefer to get them to the right length. Ideally they should be an inch or two under your hitch when coupled up.

 

I had the same thing happen to me. Mine was with a 7k lb travel trailer. The chains did their job and kept the trailer behind the truck. The jack grinded into the ground but did not rip off the trailer. The bottom of the jack did not have a foot on it so it was difficult to actually lift the front of the trailer since the bottom was hot and was sinking into the asphalt. I did end up putting a slight bend in the jack itself although it was still functional. I got the trailer re-attached and went on my way having learned a valuable life lesson. When it came unattached, I did not slam on the truck's brakes, I slowly decelerated and came to a stop. The bumper of the truck did not take out the jack or the propane tanks, the trailer did not go under the truck at all, the world did not end. Having the safety chains hooked up properly definitely saved the trailer. My brake cable was longer than the chains for that instance. I hook my brake cable to the loops on the receiver, same place I connect the chains.