We have been living in our house for 7 years. Just recently, the toilet in our main bathroom smells just like perm solution for your hair. It is coming from the toilet bowl and gets the strongest when you use the toilet and flush. It will smell for 5 minutes after you flush it. We have a separate shower/bath combo and the tub never gets used. I read somewhere that can be a problem for the drain.

Open the tank lid and give it a sniff. Any smell in the tank? I assume you brush the toilet down under the rim and all over. The reason I mention this is because urine has a strong ammonia odor.
OK! So you keep a clean bowl.
Try this. Shut the water off under the tank and pour a half gallon of bleach in the tank. Flush and let set overnight. Next morning brush the bowl thoroughly, turn the water back on and flush everything away. Smell better now? And rum water in the unused tub to keep a seal in the trap or you'll get sewer gas and odor. Good luck, Tom