The roof is supported by the cabinets - so if someone removed any of them and left the trailer out with a heavy snow load that could well be the cause of the sag. If it was left outside a few winters ago 09/10? when we had all the snow for extended time and no one cleared it off or put something inside as you have done with the 2 x4's to give it extra support that could have also done it in. More than one egg has seriously cracked due to heavy snow load & lack of roof support - although I havent seen a cracked
Bigfoot - not that it hasnt happened though - more often than not its an older
Boler. Often people remove the iron or wood spindles found between the kitchen counter and upper cabinets not realizing they are a needed support.
Sorry have no idea as to how you might fix that.
Below is a photo of a lower mainland
boler that didnt make it through the winter of 09/10.