If the fifth wheel does not reduce internal volume in any way and you can make it affordable and give it good fuel efficiency, why not?

Only one remark if you consider the fact that the movement is due to the friction between wheels and road and you use only 1/5 or even less of the weight for that then i doubt that you can get accelerations or climb slopes.

Well, as you said, I did not read carefully enough. Please forgive me.

After reading thoroughly, I do have a few observations about your idea. Please understand, there is a difference between someone being brutally honest and someone being brutal. These comments are intended to be constructive, and I hope they are useful in your pursuit.

500kg vehicle for USD $1,000.00? That works out to about 91 cents per pound. Here in Dakota, a can of generic dog food sells for 96 cents... perhaps review your EOL costing estimate, to be certain this is a realistic figure.

The safety gear required for any enclosed vehicle here in USA is a staggering load, both on expense and weight. This vehicle will, I assume, need to be licensed, and must be in accord with any applicable rules.

Any vehicle operating on the ways will be required to carry liability insurance, at a minimum. This vehicle may be a hard sell to an insurance guy.

I agree with previous post regarding traction: one traction tire on the ground provides very little surface purchase. Last winter, I had snow in my yard for 191 continuous days. SIX MONTHS! (still trying to figure out why I am still here) Should the driving tire be in the center of auto, not too sure how I would chain it up.

Once again, these comments are not intended to offend you or your sensibilities. Best of luck with your idea...

Doorman