Here is a slightly different perspective. If the hitch and frame are rated to carry a tongue weight, it is rated for that rate whether it is bouncing or not - trailers do not glide on air. Being cantilevered does add to the effective load on the hitch but no more than the 2000 lbs from the front half of a trailer being pitched forward on the hitch from a dip in the road. Those affects would be taken care of in design. In a static sense, 500 lbs on the hitch would add 500 lbs to the axle plus with that wheelbase 250 to 300 lbs that the front axle became lighter. Again, if the manufacturer has provided the hitch rating, these extra loads have already been considered.
The problems in this scenario are finding a carrier that can handle 400 lbs and not weigh more than 100 lbs itself - if your hitch is rated at 500 lbs.
Also, if the rv is pictured in the signature, the short wheel base will accentuate the affects of the 500 lbs on the hitch. Be watchful for loose steering.
