No, sorry but you cant plug into it and extend to another panel in that manner.
What you could do is do away with the first RV panel and extend the feed for it to your new one. You would still need a junction box there at the original location.
Another less desirable but legal thing you could do is open up the first box and using appropriate connections, extend the feed to the next panel. Obviously you could not use both locations to their fullest capacity at the same time but it could be done legally.
Or you cold just use a long extension cord. I bought a 50' 30A extension cord for home use.