So far it’s not been so easy to curve foam surfaces , by using the kerf cuts ... which takes a lot of time , makes a huge Mess,and generally is not much fun to do , but looks Super when finished . 
Having curved thinner foams in the past for model planes , I know it can be easily done by the heat method
... The idea is to get the area to be curved up to 190F degrees and the foam becomes bendable .. and stays that way forever. (until re-bent in the same manner) I’ve thought a long time about how to bend 2” thick sheets of foam and how to heat up a large area uniformly ... And then it hit me ...
I didn’t want to heat and bend large areas !!!... Thinking of how I’d bend a sheet of foam (4X8) .. it would usually be bent/curved in the long direction to allow for front caps , curved roofs, hatches, etc. But that’s Not really any good eh ... unless we’re only building 4’ wide trailers... 
Now if we cut a foam sheet into 2’ (or less) wide X 8’ long sections, we can have easily bent/ formed pieces , heating only a 24” wide area (easy) making the bending easy , and allowing sections to be stacked together for Wider trailers ... The narrower pieces so much easier to handle , and the foam , being homogeneous material (no grain) easily glues together to form a larger structure , somewhat like Legos . A simple bending form could be built of wood scraps and such , wouldn’t be that big , and could be disassembled after using .. Heating could be done with a simple HF heat gun , or two (front and back at the same time ) which would be pretty quick and efficient .
Pictured below is a simple Proven type of jig that allows foam to be bent in marked increments allowing graceful curves over long distances.. Many ways to build it .. easy to use... and in 24” sections would be downright Quick and Easy ... for Two people ...
Ps. a temporary full form of the curvature could be built to allow for repeating as many identical sections as you want ..
Please tell us all the details ...