Some holes just can't be sealed, like the bottom of propane lockers. The front levelers should be big avenues for air as would the hydraulic rams that operate the slides. Since the trailer never moves, it would be more effective to put rigid foam sheets in place to seal off the whole underbelly to the ground all the way around the trailer.
2012 F350 6.7 L dually, 2013 3800RE with 6 pt leveling, Sumitomo 17.5" load range h tires, Samsung 18 cu ft residential fridge, 8k Morryde I.S. with disc brakes. Full timing since 2012.