I have to agree that your dimensions are too small and you’ll regret not spending a relatively small amount more for the extra benefit; you’ll just end up adding infrastructure/concrete after the fact at more expense. 40 ft. you may get away with, but go at least 12 ft wide. Consider that a solid, non-muddy surface to stand on front and rear for washing or engine and genset access is nice, as is entry door activities.
Our short driveway and side garage apron approach are concrete, but the rear wheels of the coach sit on 3/4-1 inch gravel 4+ inches thick, 12 ft wide, with 4 ft of brick walkway and planting strip on one side. The extra room allows bay door swing space on both sides. Two regrets... the gravel base under the concrete was supposed to be deeper per code and we’ll compacted, but as is commonplace the builder’s subcontractor fudged (ours developed cracks), and if I’d splurged on running concrete all the way back I could roll a creeper under the entire coach length instead of just the front 6 feet.
If you’re gonna do it, do it right from the get-go and spend anti-regret money. Be proactive and hire a researched-reputable concrete contractor, pay for an overkill gravel base and proper cement blend, rebar, and depth, then be onsite for the prep to verify the base depth and drainage. If possible, prep sewer, water, and electric utilities on the correct side of the coach because they are a major convenience for years to come. Ours are on the wrong side; running sewer or water hose and electric cord under or around the rig is a pain. As others implore, don’t jump at the cheapest bid; it’s probably not the best quality job or the best investment.
It’s been 24 years since our house was built, so can’t speak to cost except to say put as much into it as you can afford - you’ll have to live with your decisions for years to come. If you elect to just do gravel, that’s fine but run it deep. 1/2” gravel will settle better than 1” or larger that tends to move sideways under coach tires, but 1” if weather-exposed resists weed growth as time goes by.
Joel