Place the boneless skinless chicken breasts or chicken thighs in the bottom of the slow cooker.
Sprinkle the ranch dressing seasoning evenly over the chicken.
Add the cream of chicken soup, Rotel, and minced garlic to the slow cooker.
Carefully stir the soup, Rotel, and garlic together, then spread the mixture evenly over the chicken.
Cover the slow cooker with the lid.
Cook on high for 3 to 4 hours or on low for 4 to 6 hours, until the chicken is fully cooked and tender.
Remove the cooked chicken from the slow cooker and transfer it to a cutting board.
Dice the chicken into bite-sized pieces, or shred it with two forks if preferred.
Set the prepared chicken aside.
Prepare the spaghetti according to the package directions.
Drain the cooked spaghetti well and set it aside.
Add the milk to the sauce mixture in the slow cooker.
Add the freshly grated sharp cheddar cheese and freshly grated Gouda cheese.
Stir until the cheeses are fully melted and the sauce is smooth and creamy.
Return the diced or shredded chicken to the slow cooker.
Add the cooked and drained spaghetti.
Stir thoroughly until the chicken, spaghetti, and sauce are evenly combined.
Taste and add salt only if needed.
Serve hot immediately.