Cook pasta in salted water according to package directions, reserving 1/2 cup of the cooking liquid. Drain well.
While the pasta cooks, Brown and crumble the sausage in a large nonstick skillet. When no pink is left, remove it with a slotted spoon and set it on a paper towel-lined plate.
Wipe skillet clean and add butter. Place over medium-high heat.When butter is melted, add onion and cook 3 minutes.
Add garlic and cook 1 more minute.
Sprinkle flour into skillet and cook and stir for 1 minute.
Whisk in heavy cream and half-and-half. Bring to a simmer and then add Rotel tomatoes and diced tomatoes. Simmer for a few minutes.
Add cream cheese and Mexican cheese and stir until melted.
Add Cajun seasoning, salt and pepper.
Return sausage to skillet. Add pasta and mix everything together. If skillet isn't big enough, transfer everything to a bowl to mix together.If sauce is too thick or the pasta is too dry, add in the reserved pasta liquid.
Garnish with parsley and serve.