Bill, here is pretty much how I have been changing coolant for a long time. This applies to diesels but specifically for our style rigs with the motor aid etc. I wasn?t sure what type of coolant was used in my MH when I purchased it but I wanted to make sure I got it all out so I could use ELC. Most of the ELC brands are good for 6 yrs as long as you put in a can of extender after 3 yrs. If you want Cat ELC concentrate then call up the local Cat dealer parts dept and ask for the concentrate part #238-8647. It was app $23/gal here so I used Detroit Diesel Powercool Plus ELC. The Detroit concentrate is part #23519397 and was app $12.50/gal. I?ve used it for a number of yrs in diesels so no problem there. Don?t forget to get the extender, you?ll need about a pint. Also don?t forget to get distilled water from the grocery store to mix with the concentrate, it?s only about a $1 per gallon.
If you have a C7 your rig is probably an 04 or 05 so should be similar to mine. I have the motor aid also and had app 8 gals in the cooling system if I recall. I always catch the fluid in a marked container so I know how much was in the system. (I use a 30 gal barrel cut in half) This helps you to know that you have most of the air removed from the system when you finish. Under your drivers side frame rail below the engine you will see the 2 ea. 5/8 hoses that will run up front to your heater. At this location they will have drain valves in them and this is the lowest point in the system so drain it here and then close the valves. You don?t need to unhook any other lines. Go up front and find your heater core hoses, 2 ea 5/8?. Right where the one connects to your heater valve, unhook it and add a splice with the T-fitting to install a flush fitting like you do on a car/truck. Hook up your garden hose, Install the spout on the radiator and you are ready to flush the system. Turn your hose on slow, you can easily over pressurize your cooling system if you get impatient. It will take a few minutes to fill the system and start flowing out but as long as you have flow out of the top of the radiator you are good. When it runs clear shut off the water, open the low point drain valves again and remove all the flush hardware and reconnect the heater hoses etc.
The hardest part of doing this job is getting all the air bled out of the system so you don?t end up with a hot spot. Take your time filling the radiator, go slow when pouring in the coolant. I don?t remember what it is but Cat actually has a spec to follow for how long it should take to put in a gallon of coolant. Just don?t be in a rush and you will be fine, this will allow air to come out as you pour in the coolant. Once you fill the radiator and it won?t take any more after sitting a while then go in and start the engine with the radiator cap REMOVED. This part is pretty important. Run it at 1500 rpm +- for about 3 minutes, then come down to idle for 1-2 minutes and shut if off. Wait a few minutes and top off the radiator and do this procedure once more. Then close up the system and warm it up to oper temp, let it cool and check the level in your expansion tank again. You should have added app the same amount as you drained.
Now would also be a good time to change your rad cap, run about $8 for a 13 psi cap, bring it wih you to the dealer when you get the coolant. Sorry for the length but hope it helps.