Great idea for a thread!
Using residual heat can save a lot of energy! I do it all the time when I'm backpacking and fuel weight is important. (I'm an ultralighter so sometimes I measure in half-ounces...it's an intellectual challenge.) I boil the water and add it to whatever the food is, put it in a thermal pouch (like reflectix or close-cell foam), and let it cook passively.
One way to use residual heat in an RV is a
Saratoga Jacks Thermal Cooker. I bought one but haven't used it yet. You bring the water to a boil and put your stuff in there (for a stew or something similar), then close it up and let it sit. A dinner that normally requires simmering all day will be ready for the propane cost of bringing the water to a boil...and doesn't keep putting that heat into the RV. Could be a cost-saver at home, too.
I also got a
Kalorik Stainless Steel Egg Cooker for making hardboiled eggs here in Colorado at home. We've tried all the tricks for good boiled eggs and nothing works...we always end up with the shells sticking to the eggs, and the egg rips apart when you try to peel it. This works perfectly every time, and it's small enough to take in the RV. Seems pretty efficient but I haven't measured it.
For folks who are interested in ACTUALLY using the sun too cook, google "solar cooker" for lots of plans. You can actually make an oven that will cook food with sunlight. Obviously wouldn't work well on the move, but it works.
Can't wait to see other ideas!