RPM of what - motor or a wheel/shaft on the vehicle? Cheap and simple would be to repurpose an automotive sensor. All today have crankshaft and camshaft sensors, plus similar on the transmission. Most crank sensors measure teeth or slots on steel parts (often termed "toner ring"), so are pretty general, and have simple mounting. One suggestion is the crank sensor on my 2002 Chrysler T&C w/ 3.8L engine. It is a Hall-effect type, which gives a 5 V square wave output that is DC accurate. 3-wires w/ one being 8 VDC supply. Others, like the Ford EDIS (late 1990's) sensor are "variable reluctance" so just 2-wires and a transient AC signal that has no amplitude at very slow speeds. Some cam sensors rely on embedded magnets on the rotating part. Transmission speed sensors bolt from the outside and sense toothed wheels.
I agree that a shunt resistor is simplest to measure DC current, and also most accurate. Look at Emco, etc. If you can't be in series, there are Hall-effect sensors which you pass a wire thru. Removable clamp-on types are less accurate, and tend to give different phase shift each time you clamp them. Correcting for phase shift is very important when calculating motor power, especially at low power factors. A shunt resistor should have no phase shift. When you use a voltage attenuator, be careful because it high resistance in combination w/ the cable capacitance on the signal side can give an appreciable RC low-pass filter and thus phase shift to correct.
