Electric is too inefficient
Electric is a very inefficient way to go, unless you go with motorcycle, outdoor, hunting gear/clothes, ie hot socks, liners and so on. There is so much wasted heat going out the pipes you should use it. IF you work out the Watts needed you will need a BIG alternator.
Why do we still get cold?
-Inefficient heat muff or too small
-terrible cockpit ventilation:
1) Air leaks IN causing drafts and causing cabin pressure to be too high for airflow from heat muff and thru cockpit
2) Not a sufficient VENT or air exit out the cabin allowing controlled flow
The key is have a dedicated cabin exit or vent (as high and far back in the cabin) to draw the air out in a controlled way and pull air (from heat duct into the cabin). Look at a Bonanza or Piper Comanche, you'll see exit vents in the tail (reverse scoops). A good place on the RV would be belly a few feet behind the trailing edge.
You want something to suck the air out of the cabin. If the cabin is totally sealed the cabin heat air will be stagnant and not have any flow (aka heat transfer) into the cabin.
Reduce extraneous leaks into or out of the cabin. Cold air leaks in is obviously bad but leaks out are also a problem. Leaks out in random places is not a good way to plan VENTALATION. For one it is draggy when cabin air "plumes out" in random places into the free airstream. Also the volume and location of air leaks out is not sufficient or conducive to comfort.
You want the HOT air from the heat muff to be pulled from the heat register across you feet, torso, head and shoulders and than out the cabin, not swirling around in miscellaneous places in the cabin. Here is thread on a dedicated cabin air exit vent.
http://www.vansairforce.com/community/showthread.php?t=9847&highlight=vent+cabin+scoop
I don't think louvers in the tail access panels work, for one there are so many leaks INTO the tail that it tends probably fight or negate the venting thru the tail. I guess if you ran SCAT to the tail it might be better but that is expensive and adds weight.
Insulate the cabin as best possible, although we have a full bubble canopy which makes it difficult to keep from losing some heat. If its cold but bright sunny, heating is not a problem. Overcast or night and cold, it is a challenge.
Increase the efficiency of heat muff. I had heat studs spot welded onto the pipes. This is common certified aircraft stuff. It gives more surface area. Either
AWI or
http://www.aircraftexhaust.net/ can do the heat studs, I recall. Here are my pipes. you can see the studs. The outer cover of the heat-muff which covers the studs is off, but when installed it looks like typical heat.
I guess the old stick to the plans and don't reinvent the wheel comes to mind. You can fly very comfortable with heat muffs if you design it properly. If you get on a the Vari-Ez and Long-Ez sites you can look up the viability or lack of viability to this.
I have a cigarette lighter jack and can plug in a little portable ceramic windshield fan heater for defrost but it draws about 12 amps. That is a big part of my 35-45 amp alternator's capacity, and that is for a little crummy windshield defroster.
They way they did it in my old twin was a gas fired heater blower in the nose. You just could not vent hot air from the wing engines to the cabin efficiently.
Staying warm when it is way below freezing in a metal plane going almost 200 mph is a challenge. I think hats and gloves and yes even electric motorcycle/hunting clothes might do the trick.
The last TRICK is the good old low tech blanket. For the ladies who are cold a blanket is nice, and if you can pipe some warm air under the blanket they can get pretty toasty.
