Will it weaken the signal? Not so you will ever notice.
in fact that's how I suggest you do it RG-6 (i think) is the cable recommended.
There is another way many do it.. That DOES weaken the signal. you are doing it right.
one suggestion UNPLUG the receiver before disconnecting cable from antenna and RV and don't plug it in till you are all cabled up.. Receiver powers the antenna via the coax and if you mess up and short the center wire to nut. it can poppa a circuit (Let the magic smoke out, and without the magic smoke, it no work no more)