Unfortunately, ther is now easy way to get a high R-value when insulating an older mobile home. Current new mini-homes usually have an R-20 wall rating. To acheive that rating would be almost impossible without changing the wall size to a thicker wall. For warmer climates a value of R-5/6 can be acheived by adding hard foam sheet insulation or equivalent such as glass-clad and then vinyl siding for weather protection. For higher values you would in addition to the exterior insulation need to insulate the interior wall cavity with something like R-12 bats. This would require removing old wall panels, furring existing 2x3 walls with 1x3 strapping or 5/4 flooring to get the necessary 3 1/2" width for the insulation. Then drywall or put the panel back on. Do not compress the insulation as it looses its insulating value. Then replace panel, or drywall(preferred). The roof can only be properly insulated and ventilated(very important) by building a standard peak style (or other style such as Mansard) and insulating i and ventilating in the normal way. As I said, unfortunately there is no easy cheap way to do it. Also, don't forget about windows and doors.