It works if you know values well enough and can afford to tie up the money. $20K is not much to start with, at most one unit way below the market level most buyers will be looking for.
A friend recently bought a $3000 to $4000 RV (30 foot motorhome) under $1000, as a "distress" sale, but the reason it was distress is because almost nobody is buying that class of RV and the heirs had been trying to sell it for a couple of years, had to do something to close probate.
Many small used car dealers are doing what you are talking about, but to make a living for a family they need to keep $100,000 to $500,000 tied up in sales inventory, and they are making most of their income on commisions from financing and service agreements (calling them warranty or buyer protection plan) rather than margin on the flip.
Unless you are better than average finding bargains when you buy, to make much on the flip you may have to add perceived value. For cars, you need to really understand cars. For RVs, you need to understand RVs. I know someone in theater tech who does well flipping theatrical lighting and sound equipment, but he would lose his shirt on RVs because he doesn't understand the costs of making one marketable. I say this because I've seen his first RV project sit for three years now.