Hi Tresa,
If the failure of all of these devices was that they acted as if they had no power, that would strongly suggest some type of electrical problem.
3rd party post warranty agreements typically say that they cover manufacturing defects that show up after the warranty expires. They're a type of service contract where you prepay. The issuer typically has a bunch of fine print that restricts what is covered and has a number of exclusions. If you change companies, the new policy will probably have at least many of the same limitations. I don't think any service contract (extended warranty) is going to cover external causes like power fluctuations, or freeze damage for example.
It's more likely that the comprehensive insurance on your trailer would cover external issues like power surges. The problem is that the insurance company may want some evidence or expert opinion that the damage was caused by something other than a manufacturing defect, which they wouldn't cover.
If the power company (or your brother-in-law) had determined that a nearby lightning strike, or wiring problem at the park had caused all these failures, your insurance company would likely pay. If your shore power cord showed some type of damage, that would probably be acceptable evidence.
Btw, if the power company checked things for 10 minutes and found no problems, their findings (and letter) would not mean that there had never been a spike. Just that they see no evidence that there had been one. There's a difference. And one serious electrical event could have caused damage to the power supplies of the various devices, weakening components, setting up a later failure.
If you trust your brother-in-law's skill level, you might ask him to check all of your trailer electrical connections from the pedestal to the circuit breaker panel. Pay particular attention to the transfer switch. I'm sure he knows that a poor electrical connection anywhere on the neutral wiring (including pitted contacts on the transfer switch) could cause the types of problems you've experienced. The transfer switch is where shore power connections and generator connections come together so that one or the other feeds power to the circuit breaker panel. It's usually located behind the pass-through basement storage wall.
And as others have mentioned, using an Electrical Management System like a
Progressive Industries EMS-HW50C is essential to protecting your trailer from electrical problems. The one linked to is a hard-wired model that would be installed just before the circuit breaker panel. We use an external model that has detected transient power problems many times, at multiple locations, protecting us each time.