Reality is you have such a big truck compared to such a small trailer that you can likely get away with breaking some rules without major effect. That said:
- Something is clearly wrong if you only have 360lbs on the pin. The sticker calls for 500lbs empty.
- If you are scaling at 5000lbs, that suggests around 2200lbs loaded in the trailer (a lot for such a small trailer). It could easily be that you've loaded too much stuff at the back of the trailer reducing pin weight. You should be looking at around 1000-1200lbs on the pin if the total weight is 5000lbs.
- Pin weight is supposed to be over the axle with a 5th wheel. A couple inches won't make a big difference but it's not like a bumper pull where you have to compensate for the tendency to take weight off the front axle by using a weight distributing hitch. The advantage of a 5th wheel being over the axle is it plants the axle and has negligible leverage if the trailer starts to sway (plus with 20-25% pin weight, there is little tendency to sway).
- You might get away with using the slider in the back position but without doing a detailed analysis, I would be hesitant to do so. There might be unexpected side effects (example: what happens during hard breaking, will the hitch give way and slide forward with a great deal of force?)