Thinking over the LP tanks might be a viable option, but as far as the back of the trailer I was going to use a platform style (cargo carrier) type rack where the bikes are not hanging. My DW and I always comment when we see a 5'er or TT with 4 bikes bouncing on a hanging type rack and I try to get around them as quick as possible as I don't want to run over the bikes when they fall off. I would put them on top of the Expedition but I am vertically challenged at 5'8".
As far as the extender handling the weight I was not concerned, it literally is a solid steel extender customer made about 40lbs but I was worried about the WDH and throwing off the weight distribution. I think mtnguy is right about the WDH I just wanted a second opinion.
2012 Freedom Express Liberty Edition 292BHDS
2011 GMC Yukon XL 2500 4X4 (what a difference)
2007 Ford Expedition EL Eddie Bauer 2WD (sold)