As thin as the interior wall 1/8" tempered hardboard is, I would imagine it significantly contributes to the over-all structural integrity of the motor home. But at the same time, motor home companies have massive voids in their walls to accommodate slide outs. So I would think, as long as you keep your "removal" to reasonable limits and replace & glue replacement tempered hardboard, you will do fine. But removing 100% of the interior wall, then improve the insulation, then replacing the interior hardboard will likely be a structural disaster because the replacement material is not vacu-bonded, and not bonded to virgin materials.
Like a real house, the worst of heat loss originates from windows, doors, and roof vents, so buying a quality made rig with those considerations will contribute so much more than messing with the walls. Slightly drafty windows will hands-down nix R20 insulated walls. Roof vent inserts can easily be made from styro-foam to eliminate radiated cold from those sources. The thick pink or blue foam insulation for foundation insulation is an ideal product for that because it doesn't crumble.
