2002 Toyota Rav4 Alternator Charging issue

You didn't say what those voltages are. Maybe you don't have a problem.

Problem is there:

Initially voltage is above 14V, then it goes down to about 13.55V after some time

Unless specified differently by the manufacturer, the acceptable range is between 13.75 and 14.75 volts. You're just below that minimum, but the charging system is still working. What kind of problem are you trying to solve? Are you having cranking issues? If so, all the time, or will it crank okay for a few days, then the battery is too discharged? Why exactly are you pursuing this?

The issue i am having is when driving at night time, with the headlights on etc. The A/C blower fan slows down, the power of the horn goes down etc. I don't want to end up stuck on the road, as i feel the problem will only get worse if left unattended.

A better suspect is a failed diode in the generator. There's six of them With one bad diode, the most current you can get is exactly one third of the generator's rated maximum current. 30 amps from the common 90 amp generator isn't enough to run the entire electrical system under all conditions. You need a professional load tester to test for full-load current and "ripple" voltage. Those two tests will tell if there's a failed diode.

You also must consider that three things are needed to generate current, and one of them is "movement" between the two other parts. That movement is a product of engine speed, so a slowed engine will result in reduced voltage in the electrical system. If the load test proves you're able to get the full current from the generator, it is likely you don't have a problem.

Ok Thank you. I will look in to the Diodes, and also do a load test on the Alternator.