Need some clarification. Is your shell completely off, and your frame separate, waiting for the subfloor to be installed? Is the shell still on the frame, but there is no subfloor? Is the floor in, but needs to be bolted down? Is your belly pan installed or off?
General answer is that there should be bolts that will go down through the channel that is attached to the bottom of your shell, through the subfloor, and then through each outrigger (actually, they come up from the bottom--hence you will need access to top and bottom). The subfloor also attaches to the cross members, usually using self tapping screws. Screws are usually installed every 4-6 inches. You will probably find some of the cross members that do not go all the way to the level of the subfloor, these are places where two pieces of subfloor come together, and the gap is there for you to install a ~4" wide piece of plywood to connect the two pieces of subfloor together.
If the previous owner removed all the subflooring without removing the belly pan, then you have some extra work to do. Either you need to completely remove your belly pan, or you need to cut enough of the center section out so that you can replace the insulation under the floor and get access to the underside of the outriggers, in order to run the above mentioned bolts through.