Find the coach you like and can afford, where ever the radiator is located, you can live with it.
I do all my own maintenance work and have never found the rear radiator to be an issue. Yes, it can get dirty if you overfill the oil and let the slobber tube blow that oily mist out right in front of the radiator. but that's easily avoided and easily cleaned.
Where ever the engine & radiator are located in a motorhome, doing mechanical repairs is never convenient. whether its gas, diesel, front, rear, side, matters little in the big picture.