A small sump pump. They are available at most hardware stores, Northern tool, Harbor Freight supply and probably Wal-Mart or Amazon.
Prices start around $40.
https://www.walmart.com/ip/Superior-...oaAgunEALw_wcB
Something like this might work for you. It would depend on the size of the fill opening on the top of your tank. No priming necessary, Drop it in and turn it on.
You might have to reduce the size of the hose or install a valve to slow the water flow down or the discharge of a pump like this (or any other) may be faster than the RV tank can handle causing an overflow condition.
Lynn