Just got two 12v DC pipe elbow heaters rated at 7.5 watts, 0.55amps each. I was hoping to wire them into my WFCO w8735 converter so I can run them on and off grid. There are no empty circuits, but one of the 15amp circuits only has our bathroom and 'bedroom' lights on it since we don't have the optional slide. All the lights are the LED pop lights so the draw should be minimal. would it be wrong to splice the elbow heater onto the DC output wire for that circuit? Also, would I just connect the negative wire to the busbar that they run all the negative DC loads to? There will be a 15amp inline fuse and a 3 pole toggle switch wired before the converter. Here are some pictures of the converter. https://i.imgur.com/Peix689l.jpg https://i.imgur.com/299TE0yl.jpg https://i.imgur.com/v5ek00Ml.jpg
Since you are tying into a line that is already fused at 15 amps, there is no need for an inline 15 amp fuse. You are already protected. It is perfectly fine to use the same busbar that all of the other negative wires are wired to. (You can't post links until you have some specified number of posts. Keep posting and it will work in the future.)
Here are your links as pictures:
Bobbo and Lin 2017 F-150 XLT 4x4 SuperCab w/Max Tow Package 3.5l EcoBoost V6 2017 Airstream Flying Cloud 23FB
From the sound of it yes You are looking at about one standard incandascent ceiling lamp bulb total so if you switched from incandascet to LED you have plenty of power.
Side effect. If the fuse pops or fails for any reason.. YOU WILL KNOW IT SWIFTLY and for that system.... Major benefit.
Home was where I park it. but alas the. 2005 Damon Intruder 377 Alas declared a total loss after a semi "nicked" it. Still have the radios Kenwood TS-2000, ICOM ID-5100, ID-51A+2, ID-880 REF030C most times