generator wierdness

I've got a 93 Holiday Rambler Endeavour with a Onan Marquis 7000 and two Dometic Duotherm roof ac's (front and back). In the past I've been able to run both at the same time as it should be. After not using for several months I fired up the generator and tried to run both ac's. I would turn on the first one and it would run fine. When I would try the second the generator would die. That was yesterday. Today I tried again and the front ac wouldn't start at all. There is a switch under my counter that lets you pick which ac you want to use when using shore power. As a last resort/experiment I tried flipping that switch to the front unit. Lo and behold that reversed the situation: I could start up the front ac but not the back. In other words it's behaving like it normally would when hooked up to shore power, where you can only run one ac at a time, but I'm running the generator so I should be able to run both. Hopefully my explanation was clear. Any idea why this would be happening?