Towing a Car behind a 5th Wheel

I doubt that your 5th Wheel is rated for towing that much weight, especially a car and trailer/dolly. Besides what kind of breakaway system would you use if it became disconnected and started rolling down the highway?

Do you have a hitch on rear of 5vr??

If so check it's tongue rating----------probably 300#

If not....check with your 5vr mfg. to see if hitch is recommended.
Not all mfg. will recommend due to frame structure/build.

Then go to 'towing laws by state' and check double tow

It also depends if the trailer frame is stout enough. One of our fulltiming friends "lost" his Explorer, that he "used" to tow behind his 40' Teton. No damage to speak of other than a frame member that ripped off the fifth wheel and the tow-bar.

It is legal in 39 of the lower 48 states. the "main" limiting factor is total length, which varies from 65' to 75' in the lower 48.

It may be legal in some states, mainly in the West. IIRC, there is ONE of the smaller states along the Eastern Seaboard where towing RV doubles is legal, but I don't remember which one. It is VERY state specific, and Reciprocity Agreements do not apply. You will have to check with the appropriate authorities in each and every state you enter. DO NOT rely on any list of towing laws on the internet. They have been repeatedly shown to be inaccurate!
Some people say they do it all the time in states where it is said to be not allowed.
Good luck.