The problem is not finding applications. The problem is cost and safety.
In space they make sense because as you go far enough away from the sun, solar power is negligible. Since it costs a lot to launch, the cost of the RTG itself isn't such a big deal. And you only have to worry about safety for a little while.
Relying on the thermoelectric effect, they're horribly inefficient. Here on earth we have other options that are more efficient, cost less and is way safer.
In space they make sense because as you go far enough away from the sun, solar power is negligible. Since it costs a lot to launch, the cost of the RTG itself isn't such a big deal. And you only have to worry about safety for a little while.
Relying on the thermoelectric effect, they're horribly inefficient. Here on earth we have other options that are more efficient, cost less and is way safer.