That is a huge window and support on the kitchen is a good idea even without it. Realize there is a natural tendency for the trailers to sag downward. So interior cabinetry is critical to minimizing or even eliminating sag. Yours won't be the first. Plenty of others have added support to the kitchen.
Some owners even hang stuff off the upper kitchen cabinets, encouraging more sag (like toaster ovens or even microwaves).
Essentially, as the sides bow out, the roof drops. Kind of happen in tandem with each other. I bought a house with this problem (in that case, it was an expensive fix!!)
I think it is something to be concerned about, but it should also be a relatively easy fix. You may be surprised how thin the outer skin is on these old molded trailers.
If you look at interior pictures of Bolers, you will see some with kitchen cabinet support and some without.
Many buy the support from Scamp
and modify to fit. Others use a chrome pipe, or whatever, to do the job.
I'd also see if your roof is sagging (probably).