That's a strange problem. I don't think the Grey Wolf has an electric water heater. Is there any AC item other than the air conditioning, microwave, converter, refer and television? I can run my entire trailer without overloading a pair of Honda 2k, which are about the same units, and it works fine unless someone fires up a toaster or hair dryer, etc.
Do both Yamahas speed up to full power when a load is applied? If not then the parallel cable is faulty. If they do then there has to be one or more items in the trailer faulty enough to cause the problem, and that's not good even if hooked up to shore power.
At some point during every day you suddenly realize nothing else productive is going to happen the rest of that day. For me, it usually occurs around 9 am.
Vengeance 25V (the Harley needed a home)