Bad smells are normally from drains. Most here, if have any, use bell type trap and these become dry if you do not keep BR floor washed every day or two and will then allow smells to enter from gray water (which may be black septic tank in some cases). They can also become full of hair and prevent trap - remove top cover and clean - and make sure no cracks in the bell attacked to cover. Quick check is to cover drains with water filled bags and see if odor is reduced.
Normally tank full will result to slow/incomplete flush. Most tanks only require pump out every 5-10 years (and could be even longer) but as said simple process if use government workers and not expensive. Beware if have to use private however as they tend to be cowboys.