The most likely cause is settlement. Repairing cracks in wall or ceiling drywall (sheetrock) is not very difficult. Before you work on the cracks it is usually best to relevel your home.
All structures settle and most framed buildings show evidence of settlement after as many years as you have owned this home. In most soil conditions after eight years the vast majority of the settlement has already occurred. So once it is releveled even though the settlement continues forever, it should be so slight as to be unnoticeable
Happily it is not a very big undertaking to relevel most manufactured homes. If you look under your home at the top of the foundation piers you will see pressure treated wood shims. These can be tapped in to raise the support of that pier and backed out to lower it. Most professional installers use a water level that stretches the entire length of the home and adjust each pier to bring the home into as close to level as possible.
The operations involved in jacking and raising the home can be very dangerous and should be attempted only by experienced individuals.
Take a look at The Manufactured Home Releveling Kit from Aberdeen Repair
This includes a water level along with complete step-by-step instructions. Use these instructions and the enclosed waterline level to determine whether or not your home is out of level. You can use this information to become a better-informed consumer and give yourself the upper hand when it comes to hiring a contractor to do the job.
The detailed instructions will show you how to do the following: learn the causes of why your home becomes unlevel learn how to set-up, calibrate and read a waterline level, determine where your home is unlevel and by how much learn what to watch-out for before, during and after releveling your home learn proper jack placement learn how your contractor should safely jack-up your home.
You want to relevel the home before you start patching the drywall because releveling operations may very well cause additional cracking of the drywall on the walls and/or ceilings. While a wood framed structure has the flexibility to rack out of level the drywall has almost no flexabilty. So like uneven settlement, racking the home back to plumb and level can cause enough stress in the sheetrock (drywall) to make cracks appear.
Cracked drywall alone is not a structural unsoundness. Drywall cracks require little skill to repair and only a few simple tools and supplies. For detailed instructions see How to fix a crack in drywall