I replaced the battery in my van about a month.

Measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, replace the generator again. If it is within that range, have it professionally load-tested. If the generator can only produce exactly one third of its design current rating, it has a bad internal diode. 30 amps from the common 90 amp generator is not enough to run the electrical system under all conditions, so the battery will have to make up the difference.

Are you just jump-starting the vehicle, then driving off right away? If there is a bad diode in the generator, you aren't letting the battery charge up nearly long enough. The engine may start but you're still running on an almost-dead battery. It will take at least an hour at a slow charge rate to fully recharge the battery.