Easiest way to settle this and any uncertainty about the meters is to put a terminating load on the output. That should eliminate the variable of the antenna/cable and determine the veracity of the RS meter vs the Galaxy. If both read true with the load, the RS is obviously on the money. There's plenty of cheap 50 ohm dummy loads available to take a momentary key and don't need to be continuous duty for the RM amp.
You should own one of these anyways as an essential diagnostic tool along with your watt/swr meter. One of those ribbed heat sink models in the non gold pl-259 that's rated 15W continuous duty is ideal for this. I carried a $15 Diamond clone sold by radio shack in my field bag with an N to SO-239 adapter for years in my field case for testing commercial base, mobile, and control station up to 90W from low band to 800 mhz with a bird 43 before I bought a dual use 35W Telewave that works on either connector. Well worth the investment for under $30 bucks or much less used on E bay. You don't need a high wattage unit for a momentary key to make a reading. I'd suggest making your own if you don't want to spend the money, but $30 isn't going to kill you for a lifetime investment for something you should own anyways.