If it has the standard Keystone layout, the black and bath hoding tanks, plumbing and wiring are mostly behind the bulhead seen when in that cargo (basement) area and directly below the bathroom. These bulkhead panels are easily pulled out after removing a dozen, give or take screws including the plastic water access cover. This will give you (him) a good way to see what's there. Alternatively, the belly bottom Coroplast cover can be partially pulled down. Some, depending on year, have screws holding it in place, others have a kind of rivet that can be easily chopped off then replaced with some hefty self drilling screws.
With that said, our 2014 Keystone Montana doesn't have much of anything between the compartment floor and the belly cover. If yours has a washer dryer, the water and drain lines are probably overhead on the right side. There may be a couple clearance lights that may have some wiring though that will not be attached to anything.
I would just screw the runners down with little worry about hitting anything but maybe a cross member
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011
Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014
Montana High Country 343RL (38')w/disc brakes