before you get to crazy cutting into things make sure there is enough room for access in the first place. I would shoot an email to the manufacturer first and see if they would give you something like a section drawing that would tell you if you can even fit inside the space. is the ceiling vaulted? does it match the roof pitch outside? access through the soffit is unlikely because the heel height, the distance from the top off the wall to the top off the truss vertically, is likely less than a foot and most of that distance will be full of insulation. you could straighten out a coat hanger and poke it through the drywall perpendicular to the roof and see how far you have from the bottom of your trusses to the roof sheeting. I could be way off here and maybe you have a huge attic but my experience with modulars and the lack of an access leads me to believe you wouldn't be able to fit in there.