You can do a lot better at gel coat if you get the proper products. Go to your local marine shop and ask them about marine polishes and compounds. Trust me it will save you a lot of time. Also try a few different brands of the most aggressive wool buffing pad you can get, its amazing how much better some work than others. I love doing boats because I can make 4X the cash in the same amount of time. In my area it is $180 for an exterior detail and 2 step correction plus sealant on an average vehicle, which can take up to 8 hours. For your average 20 foot speed boat its $350 for the same thing and it takes me about 4 hours. If I was still using my automotive products on boats, I wouldn't even take any of that business because it wouldn't be worth my time and the cost of advil to keep me going lol.
As for the RV I have never had a single customer with an RV. For the price I have to charge for that kind of labor people just think you're ripping them off when you quote them. They don't even like the idea of paying $2/foot for a wash! I've seen people pay a couple neighborhood kids $20 between the two of them for them to slave at washing the thing all afternoon with a hose, bucket of soap, and a brush! You may have more success in this than I have, just saying.
I have contacted some of my local flight clubs and airport services and nobody seemed to think there was any such thing or a need to detail an airplane. This is something I had looked into because I absolutely needed more business at the time but I'm glad now that nobody hired me. The thought of standing underneath an airplane wing holding a buffer over my head is not appealing to me. What Porsche Pilot said about knowledge of planes is a very good point. I have destroyed one antenna this summer on a boat with the buffer but I was able to fix it myself. I know nothing about aviation and would not want to mess with something that has to carry someone thousands of feet in the air. Thats just me though.
I believe you may have your best success with the boating scene. If you can become efficient and get great results with detailing boats, you can generate some good business. Its hard to find a good boat detailer, and I'm the only mobile one in my city except for one girl that does interior cleaning with a shop vac for $6/foot. If you can stand the labor involved, go for it. That being said there are also some very successful RV detailing companies out there. You just need to research the hell out of it, get some hands on experience some way or another, and keep sharpening your skills.
Good luck!