I am looking to fir a draw bar onto my trailer so as to make it a
little longer to accomodate a box to put my gas bottles in.

Putting gas bottles at the front is bad for two reasons. Firstly, it may tip
the weight forward too much, making it impossible to get nose weight down.
Secondly, it puts a large mass at a large distance from the axle, making the
trailer much more unstable. It is far better to distribute any large masses
near the axle to reduce its moment.

If it is a large trailer and only a small weight of gas, then you might get
away with it. However, if you can contrive a way of storing the bottles near
the middle, then this would still be a superior solution.

Christian.