We've also been doing this for years. We installed a separate plug (https://www.amazon.com/Marinco-150BBI-Marine-Charger-125-Volt/dp/B000NI38MG/ref=sr_1_16?dchild=1&keywords=15+amp+marine+plug&qid=1610989066&sr=8-16). From the plug we ran romex cable to an interior wall switch. Then we run an extension cord goes from that separate plug to the pedestal 15 amp outlet. We plug our space heater into that wall switch. That puts the heater on a dedicated circuit and runs independently from all appliances in the RV. Never had a problem. If a pedestal doesn't have a 15 amp plug (nearly all do, tho), we use a 30amp to 15amp dogbone to adapt to the extension cord. DW also uses that plug to run her hair dryer as that draws a lot of amperage.
