I am getting into this kinda late but here is my 2 cents.
Is there any chance the counter was not cut square?
Wether it is or not , can you remove 5/8 from the passenger side rear of the counter and suck in the side to square it.
When I wind up with an out of square minibarn during a build I will use the large truck ratchet straps and pull corner to corner to square the box before installing the roof trusses.
I built one out of square 1 time only and fought it all the way. It looked like crap to me when done I could see the out of square in the drip lines of the shingles but the homeowner was delighted so I said nothing. It was a cosmetic issue and the barn still stands so what the hay. However I learned my lesson and never did it again.
Back to yours if after removing the counter top the left side can be pulled out and the right side can be pulled in by placeing the rear of the trailer in a garage door and ratchet from the trailer to the garage studding. The existing counter top may be able to be cut to fit the newly formed opening if not it may need to be replaced with a new one cut to fit. As the counter top is the only structural member keeping the rear square it will have to be right wether or not it is square trapazoidal or whatever it will need to fit snuggly but without forcing. Build a cabinet front to fit as well this will help hold the sides in position. Again square or not it should be built to fit the newly formed opening after the sides are aligned.
Fix it or fight it. It will haunt you to the final piece and part.
Just my 2 cents. Bob