Can I locate the object by reading its signal with RFID readers?
I worked on a system that aimed to do just this using 900 MHz semi-passive RFID tags (battery powered tag silicon but purely passive communication, it gives you a lot more range since the tag doesn't have to harvest enough energy to power up).

ToF or TDoA (Time of Flight or Time Difference of Arrival) don't work well for RFID. You can do a measurement of phase difference between transmitted and received signal and that will give you the fractional part of the wavelength in your range to the tag but solving the number of complete wavelengths isn't possible with standard hardware (at least we couldn't find a solution). 

Our solution was to take a series of readings along one side of the field (it took about 10). The readings weren't all taken from the same height above the ground. We then took the RSSI numbers from the readings and from that calculated the tag location and height.
We were taking readings sequentially from the same receiver that was moving but there is no reason why you couldn't use a set of fixed readers.

The calculations ended up being non-trivial. You have the standard inverse square law for the signal strength but you also need to factor in the ground bounce, that will depend on antenna heights, frequency and ground material. You  get both direct and ground path signals in both directions giving a total of 4 different signal paths that are interfering with each other. You then also have to factor in tag and receiver gain patterns.

However once you have a good enough model you can create a probability map of where in the field the tag is likely to be based on the observed signals. It was taking matlab about 20 minutes to crunch the numbers on a 10 year old PC. I managed to get the end results to normally be within 20-30 cm of truth at ranges of up to 40 m.

When it worked we managed to get this down to around 5 cm error by factoring in the partial phase information of the returned signal but that was unreliable at times.

The killer problem ended up being that as soon as you added any RF absorbing/reflecting objects into the environment (e.g. people, animals or anything made of metal) then the whole system fell apart.

For the sort of problem you are describing I'd normally say to go with a UWB based TDoA based system. That will give you 10-50 cm accuracy depending on how well the system is dialed in with low latency and be fairly immune to environmental changes if base station antennas are mounted high enough. While not as cheap as an RFID tag you can make the tags in the $25 range and run them off a watch battery of a year or more.