Deep space missions might be thought of as those to Jupiter and beyond. Solar panels just don't work well at those distances from the Sun. If memory serves, solar intensity at Saturn is only about 1-2% of that at Earth, meaning solar panels would have to be 50-100 times as large to produce the same amount of electicity.

For missions to Mars that don't need a lot of electricity but do need it for months/years, RTGs are a good solution. However, they're relatively heavy for the amount of electricity they produce. As this article describes:

One of the advantages of the thermoelectric generators is their simplicity:
because there are no moving parts, they are highly reliable. Their
disadvantage, however, is that they are relatively inefficient. Thus, for
spacecraft missions that require more than a few kilowatts of power, a dynamic
radioisotope generator could be a more cost- effective solution. These
systems use the heat generated from the radioactive decay of radioisotopes to
warm a gas or a liquid to rotate a turbine. The turbine in turn drives a
generator that produces electricity.

Perhaps we need to look at small dynamic (turbine or perhaps Sterling engine) generators instead of RTGs. There's the risk of failure due to moving parts, but the advantage of much lower weight for a given power output could be very important for these missions. You might be able to reduce the amount of radioactive materials required, too. It still wouldn't be enough to placate the Luddites, but reducing vehicle mass is a good thing in and of itself.