Normally you don't bolt the snap up brackets on unless you have high tongue weight for that model Salem TT, which might have a TW of 900-1,000 lbs I don't *think* you need to bolt the brackets on? Have you had issues with the brackets that require them to be bolted on?
What brand WDH and do you have the chains vertical or close to it? A photo or two would be helpful. If the chains aren't close to being vertical, when doing tight left/right turns (like when backing up), the ends of the spring bars can move far enough to pull a bracket forward or rearward and thus cause damage to the snap-up brackets. If you can't make the chains vertical, you can move the propane tanks forward which is easy to do.
Not sure how you can snap the head off a screw like that. The A-frame tubing wall thickness is only 1/8" and I don't know how you'd break a screw head that size and not strip the threads in the tubing first?? I'd be using at least a grade 5 screws (1/2"-13) and preferably a flange head one. Did you use a torque wrench?
I think you really need to use a thread-forming screw and not a self-tapping one. Self-tapping cuts the metal and thread-forming pushes the metal aside. When using a thread-forming screw, it stays in the material tighter but it's possible to over-tighten it. If using self-tapping screws, it's possible to strip the threads when removing a screw.
One option is to buy Reese thread-forming screws (1/2"-13) like in the photo along with Reese's rivnut kit (2nd photo). The rivnuts (aka rivet nut) require a clean/undamaged 11/16" dia. hole. With rivnuts, you can readily remove and re-install bolts/screws without causing damage. The rivnuts also allow a higher tightening torque - 75 vs 50 ft-lbs.
Another option would be to through-bolt the bracket in place by using a backer plate inside the A-frame tubing with nuts welded to it. You'd cut access openings in the cover plate at the rear ends of the A-frame. The 3rd photo shows what I did for our Reese cam arm brackets.
