The last two replies mention inverters. I would guess that a 25 year old, thirty amp RV has a simple converter, which would have nothing to do with this issue. There is a possibility that the main breaker is failing, and intermittently failing to provide power. If the issues reoccurs, I would check both sides of the breaker with a meter, to confirm that power is actually flowing through the breaker. Remember that this breaker is typically wired backwards, as compared to a typical residential panel. The incoming power wire from the shore cord is attached to the screw lug on the breaker. Power flows through the breaker and feeds the bus bar, which all the branch circuits are attached to. As for a thermal reset, doubtful. As an electrician for 25 years, I have never seen one used in an application like this. A quick search online offers nothing about 120V auto reset thermal breakers used in RVs, except as a component inside of an inverter. If this was an odd situation, where there really is a thermal overload breaker in the 120V system, I would also expect to see a reset button. That said, I've never encountered a non-resettable, thermal only breaker in any 120V system. Not to say that it's never been done, but it sure isn't typical. In this case, I would be real concerned about the integrity of all the 120V connections from the cord cap to the panel box. This could easily be a heat related issue, due to a high resistance connection. I would trace the wiring, and pay particular attention to any connections and splices. The shore cord typically ends in a 4" square junction box, where it converts to 10/2 Romex. I would definitely want to tear into that box and look at condition of the spices. If the 30 amp plug on the cord is a replacement, I would take it apart and check all connections. I would also retighten all the terminal screws in the panel box.
The other issue could be a failing transfer switch, if it has one. On a lot of the older/cheaper units, the transfer switch was an option, and you simply plug the cord into a 30 amp receptacle fed by the generator.
Max, I hope you encourage this customer to have a qualified electrician take a look at the 120V part of her electrical system. I have done a lot of troubleshooting, and repair work, on 120V systems. Problems that "cure" themselves tend to make me nervous.
