02 Mazda Protege 2.0L high fuel consumption, revving 1k higher than normal while at speed.

Ok, back again with another problem with my mazda. Had a few months of relatively maintenance-free operation, but that's how it goes I guess. Anyway, noticed late last week that my fuel consumption has dropped a lot. 600k to under 500 on a tank. I did find out I need a battery (light was on while driving in to work friday morning, hooked it up to the AVR machine to confirm), but I'm not sure how that would affect mileage/rpm range. Engine is revving about 1k higher than it should. Not at low speed, but on the highway. I'd usually be revving around 3k doing 100kph, but that's now up to nearly 4k. No idling issues, transmission seems to be going through all the gears, etc. Plugs and wires were replaced a couple months ago, and I did disconnect the EGR valve around the same time but it hasn't given me any problems before now. Check engine light is on, but that was for an EVAP code last I checked.

Update: it seemed to be getting better, then worse. Car was dead as a stone this morning (needed a battery). Boosted the car and drove about 80k with transmission shifting normally. Then the battery light comes on and the rpm goes up to what it was before. Tried starting from a standstill and counting the gear shifts, seems to be stuck once it hits third. Replaced the battery and alternator belt today after work, and drove home. Battery light still on, and feels like the transmission is definitely slipping...(i'm very inexperienced when it comes to transmission diagnosis, but I went from first to it seemed like second and then third with very little time between the two, almost like it doesn't even go through second gear. still no shift to fourth. Transmission fluid is not brown/black, but clear with a tiny bit of a dark tinge to it. battery light is still on after a 115k drive (didn't want to take it this distance, but had no choice), so i think i need an alternator as well. anyone want a mazda? cheap:P