Yeah, an alarm is the way to go, if you plug them all in and the breaker doesn't trip immediately.
After all, I would THINK it would be rare that they would all start simultaneously. Driveway test them, together, for a few days and return your new purchase if it turns out to be incompatible with the others.
One thing to consider: Having all these appliances will heat up your rig quite a lot. Depending on where/when you camp, this can be a good thing or a bad thing. I sure appreciated the heat my one freezer gives off this fall. But in summer...