I understand that you are concerned about Rosie's nasal congestion and I'd like to help.
Does she seem to be congested and struggle to breathe out of both sides of her nose?
Nasal congestion and struggling to breathe are signs of nasal and pharyngeal irritation.
With nasal irritation we can also see a nasal discharge which can be yellow, white, green, mucoid or even bloody in character. In some cases with long term infections or a mass we can even see bleeding and nasal bone destruction and swelling or changes in nose conformation or around the eye.
Possible reasons for these symptoms are bacterial, fungal, or viral infections, a foreign body in the nose, a polyp or mass in the nose or a tooth root infection.
Another possibility is a sterile inflammatory condition called lymphoplasmacytic rhinitis. This disease process is a chronic inflammation and thickening of the nasal mucosa that does not cause any changes in the nasal bones, and is not associated with any sort of infection. It is steroid responsive, but high doses of steroids for long periods of time can be rough on her gastrointestinal tract, liver and kidneys and can cause diabetes. Ideally we get the condition under control with a high dose of steroids and then slowly reduce the dose to the lowest possible to control the condition. Some dogs can come off steroids completely for long periods of time without symptoms returning, but most need steroids chronically at lower levels.
Because we need to use immunosuppressive with this disease we want to make sure we look for infectious causes first and rule them out, and biopsies for a definitive diagnosis is ideal.
Allergies are rarely the cause of chronic nasal congestion in dogs, dogs seem to get itchy with allergies, not nasal congestion.
But exposure to cigarette or cigar smoke or incense/air fresheners long term can lead to nasal irritation and congestion.
Diagnosis of a the problem behind these symptoms can be complicated. We may need to perform radiographs of the nose and sinuses looking for changes in the bones or full sinuses, nasal flushes and scoping to collect culture and biopsy specimens and sometimes blood titers to look for the infection (especially if we are suspicious of a fungal infection).
To check general health I would start with a complete blood count and biochemistry profile and possible thyroid profile.
If those tests come back normal then looking for a tooth root infection, a foreign body or polyp with a nasal scope and radiographs of her tooth roots, nose and sinuses under sedation would be recommended. A biopsy of any abnormal tissue could be diagnostic.
I would also check for a fungal infection by checking blood titers for fungal infections found in your area, aspergillosis tends to cause nasal congestion and infections throughout the US.
If you decide that you cannot or would not run any further diagnostics, and her physical examination is relatively normal then trying an antibiotic such as Clindamycin, Doxycycine or Clavamox could be an option. These antibiotics are good for treating tooth and respiratory tract infections.
Best of luck with your girl, please let me know if you have any further questions.