Not to downplay how impressive the Voyager probes are, but it seems they packed a fair bit of hydrazine. From "Engineering the Voyager Uranus mission":
While it was not a design requirement, the option for an extended mission past Saturn was always protected, unless it meant compromising a major mission objective at Jupiter or Saturn.
Even though the probability of Voyager 2 lasting another five years was calculated to be in the range of 60 to 70 percent -- well below NASA's usual acceptable probability-of-success threshold -- the decision was made to send Voyager on to Uranus.
After its Uranus encounter, Voyager 2 still carried 48% of the hydrazine
initially loaded into its tanks, eight-and-a-half years before.
[1]: doi:10.1016/0094-5765(87)90096-8 (can be found on the hub of science)
No, way too little sun out here. The sun is just a dot far away.
The Voyagers both use RTGs, radioisotope thermoelectric generators. Decaying plutonium, essentially.
That's for the electrically powered systems. The thrusters use liquid hydrazine, which is common for those kind of thrusters.
Edit: There's more about that in the NASA link in a sibling comment.
Solar has nothing to do with it. Voyager uses hydrazine, of which over 80% has been used up over the years. They simply use not that much of it as it's not for thrust, but for aiming at Earth more or less.
