We are full-timers (me, my wife, plus her sister), with a 43' FR Sierra 5th wheel with 2 bedrooms, 2 bathrooms, and a large living area.
1. I would not stand on top of slide. They are built to the same standards as the walls, etc., and we all know how weak they can be.
2. I'd strongly recommend slide toppers. As mentioned by others, they provide good protection to the slide top. Also, if worse comes to worse, you can always replace the slide topper if necessary. Much cheaper than repairing a slide roof, seals, repairing water damage, etc.
3. There have been a few times that we experienced strong winds. For me, this means wind speeds about 20mph and above - including one instance where we had straight line winds in the 55mph range! In our experience this occurs primarily in the wide open RV parks in southern New Mexico and Arizona. With winds like this, the slide toppers flap real hard - noisy, and also presents the risk of damaging the topper. Solved the problem by using 3/8" rope draped over the center of the topper, with a 4' pool noodle that I had previously threaded the rope through, that I locate in the outside center of the topper. Each end of the rope drapes down over the ends of the slide, where I then tie the rope ends to the slide mechanism under the slide. FYI: with extreme winds like I mentioned earlier (55mph), I didn't mess around. I just retracted the slides, adapted to the tighter quarters, and poured a glass of wine for each of us. Problem solved.
4. I use slide supports under our long slide (about 14'). This slide is in our living area, and has a full-width sofa in the slide. With 3 or 4 folks averaging about 160lbs, that is adding about 640 extra pounds that the slide mechanism has to support. Since the sofa is against the outside wall, that 600+ pounds is concentrated at the outward end of the slide mechanism - pretty powerful leverage. While the slide mechanisms are pretty sturdy, I use two supports to help take the strain off of everything. Again, I need to point out that these rigs are not built to the same structural standards of a house - weight (especially for a 5th wheel), and cost, are 2 major factors that govern the lack of "robustness" of construction.
I do not use slide supports under the other 3 smaller slides, as any interior weight on these slides is not as significant as on the large living room slide.
4a. With slide supports, it would take a pretty dramatic event to cause a significant weight shift to over stress the slide supports/slide. Since the slides are only extended when the rig is stationary, and presumably the landing gear is down, a flat tire would not affect the stance of the rig. Of course if you retract the landing gear without first removing the slide supports, and you have a flat tire, then the whole situation could be a serious problem.
A word of warning using slide supports. Don't exert excessive upward force when placing the supports under the slide. It is possible to lift the slide upwards in its' opening in the side of the rig, with the potential for leaks, and damage.
IN SUMMARY: BE CAREFUL - THERE ARE BIG FORCES AT WORK. Our rig is 16,000# loaded. I have devised a set of procedures (much light a aircraft pilot's check list), that is unique to my rig. Even with the several hundred times that I have set the rig up, and closed it up to move on, I don't want to make a mistake. Mistakes can be expensive!
Of course, even though I am intelligent, good looking, and a swell guy who is extremely modest, I have learned a few lessons (well maybe more than a few!) the hard way.
Y'all take care, eh? Fred