Spare tire and rear bunk to receiver clearance

Thanks for all the answers each of you provided. I think all together everything was answered in very adequate detail. We are using a bike carrier that supports the bikes with hoops for the bike tires to rest in and the main insert support arm kicks up at an angle to at least 10" above the receiver to the height of the tire hoops and back at least 18+" to the vertical support that comes up between the bikes. So that design and those dimensions should provide a realitively decent amount of extra clearance at driveways not disregarding that precautions always need to be observed. We may end up resorting to a front receiver on our tow vehicles (a Chevy Express Van and a Dodge Durango, both with small V8's) and then be able to set up the TM with no rear carried bikes to contend with.
In our younger days while towing a 20' tandem upright travel trailer, we had a cheap bike carrier collapse on us and we ended up dragging our bikes unaware of what was happening until a helpful motorist waved us down and pointed out our predicament. We'd ground our way 1/2 way through the handle bars on one bike and pretty much destroyed an infant seat on the same bike dragging them on the pavement for multiple miles. Been there, done that! ;-( Gotta just chalk some things up to a learning experience!
We're looking at a used 2003 2720SL and a 2004 3124KB so the spare should be under the floor by the entry door on those particular models.