Heat dissipation in a DIP Solid State Relay
"Peak ON-state voltage" is not necessarily something that will apply to your SSR the whole time it is switched on. It is listed in the datasheet so that you can account for the voltage loss your load can see, not to calculate the dissipated power.

There's a figure in the datasheet which shows you how much current is OK for a given ambient temperature. This is what you should take into account.