pumping rain water

We would love to use rain water to flush toilets and water the garden with, but is there a on demand water pump that can be used for these?

I had a camper that had a water pump that would kick in when the tap or toilet was used.  I don't have a direction to send you other than an RV supply store, but what you are looking for is out there.

I wonder if one of those inexpensive Koi pond pumps would work.
I had a little pump from Sears that I used to water my garden. I plugged it into an exterior outlet when needed and the intake hose into my pond. The outtake hose pumped 200' uphill to my garden beautifully.
I left it in my shed and it froze and burst over the winter. It was a great little pump!
http://www.sears.com/shc/s/p_10153_12605_01299702000P
In addition, wouldn't a sump pump work as an on demand pump by plugging in when needed?

Here are some pumps including 115v and 12 v.  Most of them shut off when pressure comes up - ie: the faucet is closed such as an RV type pump.

I have successfully used the 12v model in a solar horse watering application controlled by a float in a tank up hill from the pump.

http://www.grainger.com/Grainger/diaphragm/pumps/shurflo/ecatalog/N-at7Z1z0xiuc?op=search&sst=subset

if it is just to flush toilets, you can gravity feed a toilet, last i checked. So if all the toilets are below the gutter line, then you mount some 6 or 8 inch pvc pipe below your gutter and do a simple filter on it before you put it into the pipe. think i saw the above mentioned on an australian government website. they do a lot of rainwater catching there.

notes on calculations:
for a 6" pipe, you get about 1.47 gallons per foot of pipe
for an 8" pipe, you get about 2.6 gallons per foot of pipe

how to figure out how much pipe you want
pi * radius[sup]2[/sup] * length = cubic area of pipe
if you do it in cubic feet, 1 cubic foot = 7.48 US Gallons

also of note: You can do some free pumping using a ram pump . You lose some water down to move some water up. Think it might be a little noisy, but depending on where you are and what you are trying to do, you might find a use for it. Using this, you could move rainwater to a higher place and store / use the lowered water for gardening or pumping at a later date.

don't know if any of that will help, but might give you a different way of designing something in the future. there are videos online for the pump. cool thing is there are no moving parts, just the water.

We have in fact tried several of these approaches  Our Airstream is 140' vertical from a creek.  As of now we use a ram pump to bring water up hill.  At one point we put our reservior uphill from the camper to gravity feed the camper, but there was little water pressure for showers and washing dishes.  we then bought a transfer pump which gives us plenty of water but we must manually turn it off and on!!!
in the future we are hoping to build a house, fully knowing that our nearby neighbors have spent $20,000 for their wells.  we are  seriously looking into other options

cnsilver, good to know a bit more. Can not vouch for the the quality of many of these things. In my research, I keep coming across australia. They at least seem to be trying to move this stuff into mainstream. I have found a lot of diagrams and information on different systems on their websites in the past. doing a search on something like australia .gov.au rainwater pump will give you different government websites in australia. you can narrow it down to only .gov.au by tweeking the search on the search engine you choose. Some of the results I get are

Technical overview with links at the bottom
2007 guide to using rainwater & products
Rainwater Tank Design and Installation Handbook

Have a 90+ year old relative who had a tank up in the air that caught it's own rainwater. I have seen ferrocement tanks designed the same way. The house was in north carolina and the well water was hard and would curdle the soap.

Sounds like you may have a better scope on this area than I do, but sometimes someone has run across some information you have not, even if they know less. Interested in what your thoughts and findings in this area. If anything, it sure doesn't make a lot of since to purify water to drinking levels, pumping it miles, to flush it down the toilet and pump it miles again. Only a few places I know that have designed such a system efficiently. (believe Charlotte NC is one of them).

One other thing. There are several ways to drill a well yourself if you have some time. Not sure how well the systems work, but can find the info on it if you like.