Drain excess oil from the sun-dried tomatoes and set the tomatoes aside.
Place the crushed tomatoes, Italian seasoning, and minced garlic into the crockpot and stir to combine.
Add the drained sun-dried tomatoes to the tomato mixture in the crockpot, then pour in the jar of Alfredo sauce and stir until evenly mixed.
Lay the chicken breasts on top of the sauce and press down lightly so they are just submerged.
Cover and cook on high for 2.5–3 hours (150–180 minutes total cook time listed uses high setting as per CSV); alternatively cook on low for 3–3.5 hours.
About 15 minutes before the sauce is ready, cook the fettuccine in a large pot of salted boiling water according to package instructions, then drain.
When the chicken is cooked through, remove it from the crockpot, chop or shred into bite-sized pieces, and return it to the sauce.
Stir the grated Parmesan into the sauce until combined, then toss the sauce with the cooked fettuccine until well coated.
Serve immediately.