Many older mobile homes with 2x2 and 1x2 trusses just get tired and worn out as they get older. Some hold up surprisingly well. i think there are multiple factors including original build quality, quality of the wood used, excess weight on the roof from snow, 2nd roofs etc.. and what may be the prime factor, too many people on the roof doing repairs too often.
Some MH trusses are well engineered, with precise angled braces at 45 degrees. Some just have vertical braces with paneling triangles stapled and glued to them.
You can straighten your ceiling by making some type of ceiling jack. I use an 8' 4x4 covered with carpet, jacked up under a truss with load jacks, like they use in semi-truck trailers. You could use tall screw jacks, hydraulic jacks with 4x4s and a helper to hold on to it or even 2x6s cut just slightly taller than needed to elevate the truss back into it's proper position. I have found that jacking the ceiling up about 3/8" taller than needed works best. The trusses usually relax down to the proper level.
Once the truss is where I want it, I reinforce the trusses with pretty large triangular pieces of 3/8" cdx plywood. Keep the long side on the bottom chord of the truss and center the point to where the vertical braces meet the top cord. I use 7/16" construction staples and glue to hold it all together, fastening the plywood to the vertical braces and top and bottom cord. I feel the staples are important because they don't require banging on the trusses or possibly splitting the original truss wood as screws can do.
That's the way I do it anyways.
Today is PERFECT!
All information and advice given is for entertainment and informational purposes only. The person doing the work is solely responsible to insure that their work complies with their local building code and OSHA safety regulations.