We actually don’t pay for water in the trailer park, (its included in the rent) and RVs use very little water anyway by their nature.
On the other hand, CA is in a drought (again), one never knows when the next earthquake (or revolution perhaps?) might cut off the municipal supply, and I wanted to start gardening without increasing how much water I used.
Step 1: Rainwater Collection System on My RV Trailer
(Most of you probably live in houses instead of RVs, so you can skip all this and just tap into the existing rain gutters)
Since the trailer doesn’t really have gutters, I started by applying a thick bead of silicone around the perimeter of the roof, except above the slide and the awning, so that the water will flow to those areas. More silicone on one side of the slideout section, and the awning drains to one side because of how it is tilted.
I built the spouts from layers of aluminum tape so I could shape it precisely the way I needed.
The tape feeds into a funnel (mesh covered to keep out the crap), which goes into PVC pipe.
On the slide side its basically just a long piece of straight pipe.
The awning side was more of a challenge as it has to go around a number of various corners and through narrow spaces to get to the storage barrel on the other side of the house.
Instead of trying to measure and cut and join a whole bunch of short plastic pieces, I used flexible aluminum dryer vent.
Three lengths together made the perfect length. It’s supported with a bunch of random stuff, bungees,  rope,  metal bars,  and blocks of wood, taken from my cabinets, shed and scrap pile.
After the first little rain the dryer vent drooped in 2 spots, and I added and redistributed the support. 
At the end  an extra large funnel collects water from both sides , and drains into a 55 gallon water barrel. 
The barrel is used, but thoroughly cleaned and pressure tested, and was purchased locally by a company which specializes in second hand barrels, only about 5 miles from my house. A 55 gallon with a removable top (for adding a spout, and cleaning if it becomes necessary some day) cost me all of $20 (even). They didn’t have a spout kit – they said they could order one from the warehouse; but I figured I could get something from the hardware store. The book had warned me I wouldn’t be able to, and I should have listened. I ended up ordering one online.
Waiting for it to show up gave me an excuse to be lazy and not work on the system, and fortunately for most of that time it was dry anyway (I did miss some rain, which was rather tragic, but what can you do?).
I finished it just in time for the season’s last rain, making it the first time in my life that raining actually made me happy.