Adding a rear "bike carrier" receiver

Have recently bought a used '07 2720SL. It was a bit neglected, but we got it for a decent price and are getting it cleaned-up (not too difficult) and trying to find a ceiling leak to get it sealed (a bit more challenging).

Anyway, one feature that did not come with this unit that I would really be interested in pursuing is the rear receiver for a bike carrier or similar. Anyone familiar with these things? I took a look underneath the camper at the back end where the receiver would be, but see no part of the frame or anything where something like that would be attached. Do they sell the receiver assemblies for the units that can be added-on later, or is it something that really can only be installed at the factory (in other words, does the phrase "forget it, kid, you're out of luck" apply here)?

Advice welcome and appreciated.