It might depend on what the floor covering is over the heating wires. I'd think 30 minutes should be plenty to detect heat, perhaps not really take the chill out of the RV, but at least know they're working. If you're hooked to an electric meter, does it show an increase when the heat is on?
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI