Same way you use a propane heater in a fish house for winter fishing on a lake. There is no 3 feet of clearance all the way around the portable heater. It simply uses radiant heat to make you feel warm but they don't heat the entire fish house.
If you want to heat the entire RV try out electric baseboard heaters. They run on 120Vac and are used quite a bit in cabins during the cold hunting seasons. An electric blanket on the bed is a great way to stay warm during the night.
That 3 foot clearance is a warning label to keep lawyers happy and companys out of lawsuits. Not much else. It's not how people use small portable heaters in real life. They plug em in and get real close to stay warm.