Everything I have ever read says putting weight on the back bumper is a great way to develop metal fatigue cracks in the frame. When the trailer is in motion the frame flexes, extra weight on the back end - picture a 500 lb man bouncing up and down on the end of the diving board.... bet on the board failing prematurely!
The tongue boxes really make a lot more sense. Lots of people put a second battery, etc on the tongue.
I've got a new 22ft CCD with small steel propane tanks. I'm keeping an eye out for aluminum tanks just cause A) they are pretty enough to eliminate the cover if you want to polish them up and B) they are considerably lighter than the steel tanks. I'll be adding either a second battery or an EU-2000 generator.
Also IMHO, the best way to create extra space is to review what you are carrying and weed out stuff! I'm a natural packrat, but just this week I went through all my cloths and if I didn't wear it more than once this summer It went OUT. Sure I'm gonna lose weight and wear those jeans it takes pliers to close the zipper on....
Honest, I have lost a few pounds, but I'll buy NEW jeans to celebrate the next 10 lbs. Meanwhile a girlfriend of mine who is 20 lbs ahead of me, and on a tight budget, can enjoy those jeans NOW.
Rule #2 for freeing up space, NEVER buy at Sam's Club, etc. unless you are going to share it out with 2 or 3 other people. I've still got 3 unopened rolls of paper towels from a pack I bought six months ago.
I'm gonna sew a sampler that says, "I got this Airstream to get away from it all, not to haul it all around with me!"
Happy Trails to you, Tin Lizzie