Not being an HVAC guy can only make a guess at the real need for these grilles.
Theory!! Yes, they are needed when you operate the furnace, or you will end up with a positive pressure in the living area and freshly heated air will be 'stalled'. These then allow air to fully circulate and be recycled as partially warmed air to again be sent through the furnace for more heat and reducing the use of propane(
). At least that's my theory
Reality!! In the case of an RV which in many, if not most cases isn't well sealed. These grilles are probably not actually needed and with whatever leaks out through various gaps, access penetrations and builder mistakes would be enough to maintain a constant almost negative pressure inside, allowing heat to enter.
But with that said, I'll continue blocking them in warm weather and let the a/c keep us cool without losing any of that chilled air.
Disclaimer: This is
MY best guess