I recently purchased a prewired 30a rv subpanel to plug in a travel trailer while sitting at home. The subpanel is complete with a 30a breaker and receptacle. Is it acceptable to run the romex (10-2) to a new 50a breaker in the main panel having the 50a feed the 30a breaker?
No, you can't do that. The 10-2 romex has a 30 to 35 amp maximum depending on the insulation. The breaker in the main panel's job is to protect the 10-2 so it has to be a 30 AMP breaker, not the 50 AMP.
If this is going to be a permanent type installation you need to think about running your cable in such a way that it's not a safety hazard or a code violation. You can't have it lying on the ground running from you panel to the trailer.... maybe conduit from the panel to the outside wall and have a receptacle.... then a plug from you subpanel.
If you want to wire in 50A for a future, larger RV, then you need to run the cabling with 6/3 UF cable (not NM/Romex, you cannot use Romex because you will be going outdoors).
If you were to do that with e 6/3 cable, with the panel you have bought, that would be fine.
However, it's a common case for someone to buy the wrong thing and then go into backflips doing even more wrong things trying to make the wrong thing work. Don't bother; get the right thing. If you are mail ordering this stuff, thats a mistake - local electricians must shop somewhere; shop there.
They make RV panels which have all the breakers and sockets necessary to support all of
- a 240V, 50A large RV (with NEMA 14-50 receptacle)
- a 120V, 30A small RV (with TT30 receptacle)
- typically a 120V/20A plain old outlet for hand tools etc.
This works off a 50A breaker because it is presumed you will only be powering one RV at a time. But you still need 6/3 outdoor cable; 10/3 won't do.