Bio Filtration in a nutshell lesson....
turtles and fish produce waste. Waste is high in ammonia. Ammonia is toxic even at low concentrations to aquatic life.
There's a few strains of naturally occurring nitrifying bacteria that feeds on ammonia. The bacteria consume the ammonia and their waste is nitrite. Nitrite is still toxic to aquatic life... but less toxic than ammonia. Luckily there's yet another few strains of naturally occurring nitrifying bacteria that consume nitrite as a food source and produce nitrate as waste. Nitrate is much much less toxic than nitrite. Without having a HUGE (impractically huge) filtration setup consisting of many plants and algae... there isn't any way to remove nitrate from the aquarium. So we need on occasion to swap out water with high nitrate concentration with water containing less nitrates (clean tap water) to keep the nitrate levels in check. Hence the need for a monthly water change.
If you read up on the nitrogen cycle you'll get a much more scientific explanation than what I just mentioned.
Bio media is very simple.... it's a super porous material (usually ceramic) in the form of rings or nuggets that provides surface area for the nitrifying bacteria to reproduce and colonize. IE a bacteria home.
You'll hear about a tank that is "cycled". What this means is the bacteria has reproduced to a large enough population to consume their food source. So if you were to measure ammonia and nitrite levels in a tank, they would be 0 and stay there. What happens with a new tank is the ammonia spikes as the turtles or fish produce waste. The spike begins to decline as the bacteria reproduce. When the measurable ammonia levels drop, the nitrite levels (from the second type of bacteria) begin to climb... then spike.... then eventually go to 0 as the second bacteria have reproduced to where they can eat all of the nitrite. Then nitrates climb... and the water change. And slowly nitrates climb again... and another water change and over and over and over. This is the nitrogen cycle. A cycled tank is considered a mature tank safe for fish (not safe from hungry turtles) where ammonia and nitrite remain at 0.
Hopefully my babbling made sense. On to your filter.
I read this post earlier today and spent the day thinking about this. I came up with something....
There's a new filter on the market available at most chain pet stores called the API Nexx. We turtle keepers on this forum generally hate this filter for it's design. HOWEVER for you, it might be perfect. It's not the biggest filter and it doesn't hold much media... but you only have a 40 gallon tank.
The unique (one of the parts we hate that you may love) feature of this filter is an external pump. The pump for the api nexx is IN the aquarium external to the filter body. Most canisters have an electric pump built into the canister body.
That means you should be able to replace this external pump with any other external pump that has similar GPH flow characteristics and plumb your new pump to your API nexx filter body.
I don't know of any non-electrical water pumps.... but I do know of solar based pond pumps. What if you got a solar based pond pump, put the tiny solar panel outside a window or on the roof, and powered your water pump via solar power. You could then use the solar pump to feed the API Nexx canister filter. Many of the solar pond pumps are 12v DC. If you're traveling in a motor home... I bet you know what else is 12V DC. Run the pump off solar power when you can. When you can't you can plug the thing into your car's electrical system.
The pumps and panels appear to range in price from $10 to $yikes, but here's a 5W, 12V DC pump + panel for $66
http://www.ebay.com/itm/5-Watts-Solar-P ... 560wt_1398Ideally, you'd want to push your water volume through the canister about 3x per hour... so 40x3 would be 120 gallons per hour. However in your case none of this is really ideal... so make do with what you can. The bigger the pump the better... but the $66 pump linked is 100GPH.... probably close enough. You'll probably have to do some kind of backyard engineering to adapt the different hose sizes, but that shouldn't be anything a trip to lowes or home depot couldn't handle.