This is not jeep related.
If a machine's requirements are 240v, 4.7kw, 20 amps does that mean it takes 4800 watts?? (240volts x 20amp = 4800 watts)
Correct, assuming that it is at unity power factor. At a power factor of 0.8, you're looking at about 3.84 kW.
If it is a motor load, then factor about 6 times the locked rotor current for initial starting capacity... otherwise, you should be good to go.
I shouldn't need an inverter, should I? If you can plug into a generator than that means it already has one, right?
Technically, no. A generator is by itself an alternator driven by an engine. An alternator creates AC power. (Alternator = Alternating Current = AC power.) So if your devices are powered by 120 VAC or 240 VAC, then you simply need to wire them up accordingly. No additional 'inverter' is required.
An inverter is a device that takes a DC input and then "converts" it to AC power. You can see these all over the place... in cars, Jeeps, trucks, semis, power systems, etc... It takes your vehicles DC system and makes AC to power various devices like TV's, radios, tools, etc...
A rectifier is the opposite; it takes in AC power and "converts" it to DC power, much like a battery charger does.
The UPS that sits under a desk and keeps your computer on? It has a rectifier, a battery, and an inverter. AC ---> DC ---> AC. When the power goes out, the battery is already on and keeps the inverter running until commercial power comes back on. When it does, the rectifier powers the inverter AND charges the battery for the next outage.
Make sure to balance the loads between the phases so it runs smoothly. Most small generators like those have two "phase legs" that each produce 120 VAC each, 180 degrees out of phase from each other.
Phase A <-----> Neutral <-----> Phase B
120 VAC -------- Neutral --------- 120 VAC
----- 120 VAC + 120 VAC = 240 VAC -----
It's correct to call it 240 Volt single phase, but just remember that you are technically using both legs to get the voltage. The remaining 120 VAC devices need to be split up between A-N and B-N such that the overall loads are balanced. If you don't, then it's like riding a bicycle with a bent pedal... it's gonna shake and not be smooth.
Chris is right regarding the wiring... you don't want to make an 'oops' so be sure to consult the NEC (or a local engineer/electrician) for guidance. You'll want to focus on the whole "neutral/ground bond" and see how it applies to the vehicle. A vehicle is normally insulated from the earth via the tires, but you don't want to have a potential difference between the earth/chassis parts/human... that could create a situation where two parts on the truck are at a different potential (voltage) that could kill someone.
Using metal conduit is a good idea, and grounding the conduit will help prevent bad things from happening. Using a GFCI circuit breaker at the generator would be able to detect if you had and current from the phases not returning through the neutral, or going through the ground (chassis) or a human.