Different Inverter Question

Okay, here is my situation. I have a 2500 Watt Cobra Inverter, a med sized dorm fridge, 600 watt microwave, 4 cup coffee machine. The problem I am having is voltage drop. Whenever I use an accessory, the volts (on the dash gauge) drop from 14.1-13.9,13.8,13.7,13.6, then 13.5 and POP blows the circuit breaker. This is in a 2004 Columbia. I originally had 4 gauge battery cable run to a 150 amp circuit breaker. I called cobra and they told me the amp circuit breaker is not enuf. After looking back in my manual, they were right, I need at LEAST 250 amp. They also said that while 4 gauge is sufficient, I may want to upgrade to 2 gauge. Which I did yesterday. I am still seeing the voltage drop. I did not put a new 250 circuit breaker up there cause I am waiting to get one shipped but I ran the coffee machine just to see what it would do with the new wire. Voltage dropped to 13.1 and held steady. YES, I WILL PUT A CIRCUIT BREAKER IN LINE.
The question of the day is this....is 1 volt drop normal (14.1-13.1) when using these things or do I have a weak alternator?

 

Well, a quick read of my owners manual probably would have fixed it, but I had to call Cobra first to realize my mistake. Previously, before the 2500 watt inverter, I had a 800 watt unit in here with the same 4 gauge wire and a 80 amp fuse. Once I installed the 2500 watt, I blew two fuses in a 3 week time frame and then upgraded to the 150 amp circuit breaker. Where I got the idea that 150 was what I needed I will never know. The Cobra rep AND the manual say you need a MINIMUM of a 250 amp fuse.
Anyway, I installed a 250 amp circuit breaker and also upgraded to 2 gauge wire which was not required, but I did it anyway.
Now, my voltage still drops, but holds steady at 13.1 and it doesn't blow the breaker.