there are a few reasons to go hillbilly hilton - keeps the RV half cleaner since it's not shared with garage space. easier to tie down. easier to wash the rig on the trailer. cost: an RV toybox can't carry anything but the smallest lightest crawlers, they're barely rated for a UTV because they're intended for a couple bikes or quads. To get an enclosed trailer HD enough for a fullsize rig you're talking either BUKU bucks for something like what Ryan had or outfitting it yourself.
If you want to haul a tall or wide rig you're limited again.
but you loose interior garage space for spares, fluids, etc...