If you want to do this, you want to be fully informed on what can happen.
First, doesn't Lance use BAL frames? They don't use welding and have huck bolts instead. You would not be able to do any re-enforcing by welding. I would go to the Lance forum and see if you can get info. there and/or call Lance.
There is a definite risk with adding weight to a standard bumper. It's my understanding that the manufacturers state a max. wt. of 100 lbs on a bumper and is static weight. The rear end of a TT bounces up and down the most and some TTs are worse than others (depends on GVWR, axle rating, length and axle position). Watch the rear of your TT in the mirror some time and watch it bounce up and down. If you hit a speed bump, things can be catapulted clear off the bumper.
If you really want to do this (if you've found you can weld onto the frame), go look at TTs that come from the factory with additional storage at the rear like Nash, Keystone and KZ and see how they re-enforce things and they build the racks and what weight limits are. Go to a reputable frame & axle shop (must use a certified welder) to get any work done, not any old welding shop or neighbor down the street.
Besides structural concerns, another downside is that you can't see the rear of your TT when driving and you will not know if something has fallen off or is about to.
The owner of the TT in this photo insisted a bike and generator added to the bumper was not a problem.
