I can't speak for the coolant loss, maybe it is because of using the heater now that cold weather is here. You could add coolant/water mix to the overflow to bring it back to normal level and keep an eye on it. My own overflow did the same thing last week, I have no leaks or other problems but I had to top off the overflow. I figured it is because I have some coolant in my heater core from using the heat the other day.
The pinging is a very common problem on the Magnum engines. It is from carbon building up in the combustion chambers and on the piston tops. You can have a shop do an induction/intake cleaning service like BG or Everwear, it may help some, then use Regane every 3000 miles to help keep things clean. You can also put in a set of Autolite 3923 plugs, they are one heat range colder than stock and can help with pinging. Use the Echlin cap and rotor from Napa, the ones with brass contacts to help get better spark and a little more voltage to the plugs. You may not want to do it but you can also put in a 180 degree thermostat which will trick the PCM into thinking the engine is not fully warmed up to normal operating temperature. The PCM will then advance the timing just a little. 3923's and a 180 thermostat will stop the engine from pinging but the lower thermostat can cause a little lower MPG. I have had a 180 on my truck for about six years now and have never really noticed any detrimental effect on MPG but it may be different on a V8.
One other thing you can do is take the truck to the Dodge dealer and they can re-flash the PCM so it will retard the timing by 4 degrees. That was Dodge's "fix" for the pinging problem and it does not really work. All it does is take away some power. It can turn a good engine into a real pig, and once the flash is done it can not be reversed. The PCM flash is known as the "Death Flash" in the Dodge community and is not recommended. You can Google the pinging problem or check on DodgeForum.com for a lot more info on it.