Use tubing and plumbing to support standard half inch pipe thread connectors if at all possible. By standardizing on a common fitting, you can carry a few spares to make it easier to handle emergency situations. It may mean rather small pipes in some cases but RV's only have short runs and need to be rather stingy on water anyway. There are fittings commonly available that will work on either copper of plastic pipe and only need hand tightening for a drip free connection.
The flexible tubes found in hardware and home supply stores for connecting sink and toilet water feed lines can provide several benefits for your RV plumbing. They have gaskets and can be found with fittings that are easily tightened or loosened by hand. This makes them easy to disconnect to help drain water out of pipes for winter storage. They also absorb vibration which will help keep your water pump quiet if you use the flex lines on both sides of the pump.
PEX tubing is perhaps a first choice now that the gray plastic polybutelyne tubing has gone out of favor. PEX bends easily which can eliminate joints and fittings. It is also quite rugged and will expand to accommodate freezing which can reduce over-winter plumbing problems if care is taken to make sure that draining the pipes will at least clear any fittings that do exist of residual water. For PEX pipe and fittings, see http://www.pexconnection.com
The gray polybutelyne pipe connectors could get hairline fractures if exposed to highly chlorinated water over long periods of time. As a result there were lawsuits and a lot of insurance funded plumbing work in residences. This problem is not considered a risk in RV's as they are not usually hooked up to chlorinated water systems for long periods of time See www.pbpipe.com Most of the problems with this pipe can be avoided by using compression fittings or crimped fittings using metal barbs (the plastic acetyl barbs were the primary source of problems) and keeping the pipe away from sunlight and excess heat.
Note that copper is not a good pipe for low Ph (< 7) water. Again, this concern is mainly in household systems where there is continual exposure. Acidic or low Ph water is usually only found in household well systems in some areas.
The key to keeping plumbing from damage in cold weather is removing water from any place it can collect in a confined space. Laying pipe for easy drainage will help but there are several problem areas to watch, even if compressed air is blown through the system.
the pump - Disconnect both sides to drain completely and then allow a few days to dry. If the time is not available, pump RV antifreeze through it to fill the pump.
The toilet - careful draining is a start but you will have to rig something to hold the valves open so any residual water can dry out. This is probably not good for the springs on the valves.
Drain traps - these are designed to be resistant to draining so your choices are a dollop of antifreeze in the drains or sufficient time to dry them out.
Other pipe loops and droops can catch and hold water. Look for sink sprayers and water filters. These should be disconnected and bypassed and completely drained or removed for warm storage.
Holding tanks for fresh water or effluent should be stored nearly empty. A bit on the bottom is probably OK as long as it has plenty of room to expand and the surface area is much much larger than the perimeter area. This means the perimeter (tank walls) will not have to push very hard to cause freezing liquid to heave up in the middle.
Cold temperatures may also cause early failure of some plastics - sink sprayers seem particularly susceptible.
Batteries, Tires, Leaks, Vents (reefer, plumbing), Rodents and pests
Marshall and LittleBear suggest the Camco site http://www.camco.net/camco-mfg-online/faq for a discussion on freeze vs burst points. Also, see the Dow Chemical site related to DowFrost PG at: http://www.dow.com/webapps/lit/litorder.asp?filepath=heattrans/pdfs/reg/180-01314.pdf&pdf=true for properties of PG at various dilutions.