The positive ground is not an issue with your RV installation. You do not "ground" the panels or the controller.
The reverse polarity protection has nothing to do with it either. They are just saying it only works up to that input.
You get the same amps with series or parallel. It is just that with series you can use thinner wire from array to controller due to the lower amps. The wires from controller to battery are at 12v in series or parallel, so no diff there.
The notion that in series you get higher voltage and so it all still gets you something in low light does not seem to work out from my experience (limited). The controller loses efficiency with a greater spread between input and output voltages, negating any advantage from the higher input voltage wrt amps to the battery. (Near as I can figure it)
On balance, including with shading, IMO you are better off going parallel with an initial installation since you only have to buy the fatter array to controller wire once and it is the same installation effort. If you already have the wire installed and want to add the second panel, it would be easier to go series. But now you need a controller that can handle the high Voc input (and that in cold weather too). You are already on that by looking beyond your 50 Voc limit Eco-W 20 amper.
With a portable set-up as mentioned in the OP, there is no installation so no existing thin wire to consider. What does happen is you get shading on the panel(s) from the RV itself and whatever else is there nearby as the sun goes around and gets lower. You can see the amps go down as more of the panel gets shaded by your trailer or else you have to move the panel over so the trailer is not shading it. With parallel you still get some amps, but in series you don't. BTDT.
