To determine if it is RG-6 find the inside end of it.> RG-6 is larger outside diamater than RG-59.. It is that easy.
NOTE: You have one of two video selectors in your RV, either a box of many buttons (the jacks on the back are labeled, one of them says CABLE and that is the one you are looking for) or a wall plate with a 12 volt outlet, switch, led and antenna connection.
Remove the wall plate, and you should see 2,3 or 4 coax cables, There are a few odd ones out there but the standard is as follows
With the 12 volt outlet at the top
Antenna - Cable - Tv 2 (optional TV 3)
How to connect
Remove cable from wall plate, using a barrel connector (Double female) extend coax to Sat Receiver, Run TV-out from Sat receiver to just vacated port on switch .. Replace switch.
This will require you drill some holes to route the cables.
OPTIONAL and what I recommend.
Leave it the way it is and run a new cable.. this is the best way
NOTE: The major difference between RG-59 and RG-6 is loss.. Now TOP QUALITY RG-59 is not much different from bargain RG-6 (Source Belkin, they make both).
But HIGH QUALITY RG-6,,, IS much much, much better than even the best of RG-59, Like 1/4 the signal loss. So a SHORT run of 59 likely will nto do much damage but if the run hits like 30-50 feet, Well 50 feet is like 200 feet of RG-6.
And lowest bidder 59,, is worse.