Is a computer without RAM, but with a disk, equivalent to one with RAM?
Sure.  In principle, given appropriate hardware, you could have just a disk, with everything stored on disk.  Any time the CPU did a load or store instruction, there could be some hardware that turns that into a disk read or write.  It'd be extremely slow: on a magnetic disk, each seek takes about 10ms, so you could do about 100 random-access reads and writes per second.

Some systems have flash memory-mapped into their address space.  Flash memory provides non-volatile (persistent) storage.  So, in some ways this resembles what you mention -- though those systems usually also have RAM as well.