Shift weight towards the back of the trailer. Your heaviest items should be directly over the trailer wheels. Worst case scenario, take the gas bottles out of the storage box and try to place them as close as you can in the boot to being sitting over the rear axle of your car. You might even want to think about a battery box for your leisure battery, and do the same with that.
This might sound like a lot of faff, but it's better than finding yourself upside down on the motorway. A very real danger if you ignore your weight distribution.
If you are carrying the awning/ poles etc in the trailer try moving them to the rear of the trailer if possible along with any heavier irems. Failing that put one gas bottle in and carry the other in the boot.
We had the same problem and had to move second bottle to the rear of FC. We check the weight of the tow bar before we set off its amazing how just moving one thing from the front to the rear can get it within the tow bar maximum weight for your vehicle. Also makes it a lot safer to tow.
Depends on how you are using gas. If you are on ehu & only using gas for cooking then 4.5kg Calor will last you at least 5wks of daily cooking so no need to cart around 2 bottles.
To check contents of bottle take a small spring balance. Stamped on base of bottle will be tare, ie empty weight of bottle. Weigh bottle at anytime to check amount of gas left.
That's a good point. Most good campsites will be able to sell a refill, and many Petrol stations do as well. So as long as you don't have to be self sufficient miles from civilization, 1 should do it.