Is there a liquid which boils at room temperature and normal pressure, and can we use it to produce electricity?

Let's first talk about power stations in general. Typically, you don't really want to operate them on a small temperature difference. The reason is Carnot's theorem, which tells us that the efficiency of a heat engine depends on the temperature difference that's used to power it. The maximum possible efficiency (work output divided by heat input) is
$\mathrm{\eta <!--\; \eta \; -->}=1-<!--\; -\; -->\frac{T_{\text{cold}}}{T_{\text{hot}}}.$
For a typical heat engine used for electricity generation, the cold reservoir is around room temperature (since the power station dumps heat into the outside world, e.g. by cooling towers). The hot reservoir is provided by whatever powers the power station, but will be set up to be above the boiling point of the working fluid. (I assume well above it, as you really want it to be as high as possible for maximum efficiency.)
So for a general power plant, lowering the boiling point of the working fluid wouldn't help. It would allow you to use a hot reservoir with a lower temperature, but that's not actually what you want to do, since that would lower the efficiency of the process.
The temperatures in the equation above are in kelvin, so if the hot and cold reservoirs are 45 and 25 C, the maximum efficiency is $1-<!--\; -\; -->\frac{298}{318}\approx <!--\; \approx \; -->0.06$
Your idea is to generate electricity from the difference between night and day temperatures. With this heat source you don't have the option to use a larger temperature difference, so a working fluid with a boiling point around 25-30 C might in principle be a good idea. The resulting engine would be inefficient but not impossible to build. However, in practice, a better way to generate work from such a small temperature difference is the Stirling engine, which uses a gas as its working fluid, without taking advantage of a phase change.