So if you are old enough you will remember the comedy Green Acres. Oliver assigned every electrical device a number from one to seven, all the devices could up to a total of seven can be plugged in at the same time.
Same thing in your RV. Plugging a 2nd cord into the 15 amp cord may do no good depending on how it's wired as it may well still pop when you exceed your AMP draw.
We have a 30amp RV too, thankfully our power center shows me the AMP draw so we have a good idea of how much we're using. We generally run a couple space heaters, however I set them for the 750watt draw which is the "low" setting, it takes the edge off and the LP heater kicks in if it gets colder.
If we want to use the microwave, coffee maker, etc we shut off or or both space heater(s). That's the easiest way, just know how much you can use and manually turn things off. To give you some idea of the draw here is a
list of some common RV devices and their general amp draw amounts.
