I loaded the single axle Haulmark trailer this week with 700kg. The maximum payload is 750kg. At the supplier I was weighed on a weighbridge going in (empty) and weighed again going out, after getting loaded. Soil, gravel, sand, and compost is what they sell.
I took the photo to show how much the loaded springs move, how much the wheel travels up into the mudguard when loaded near the maximum. The movement was about 50mm.
I think that means, if you change the bodywork, if you take off the original "box" and install a full-width camper body or ATV carrier setup or something, the wheelwell needn't be half as high as the original mudguard. Or is that a wrong assumption?