COOLANT REPLACEMENTThe following information was provided by Joe Brown.  This details the procedure to change the coolant in the Cummins ISB engine.
The Freightliner RV recreational chassis maintenance manual tells you to
remove the radiator cap when the engine is cold, then run the engine until
the upper hose is hot.  I chose to run the engine just a short time so as not
to have to drain scalding water.  I chose to remove the 2" coolant hose from
the front of the transmission cooler reservoir.  The transmission cooler
reservoir is on the curb side and can be reached by opening the LP gas
compartment on the '99 36T.  If your LP gas compartment is not the next to
last compartment, you will have to crawl under the coach. It appears to be
the low point in the cooling system, and drains the system very nicely.
You'll need a container that will hold at least 10 gallons of coolant.  I
used a large mixing tray that I purchased at Home Depot.  The overflow bottle
is attached to the radiator bracket with two bolts.  You can remove the
bolts, dump the overflow bottle, rinse with a garden hose, and reinstall the
bottle with the two bolts, without having to disconnect the overflow hoses.
After the system drains completely, push the coolant hose back onto the
transmission cooler. There is no need to tighten the clamp while flushing the
system.  The hose will stay in place if the system isn't pressurized.  Don't
forget to replace the hose and tighten the clamp after the last flush, before
Our rig has a decal on the surge tank (the tank mounted above the radiator
with the radiator fill neck and cap ). This decal has better detail than the
maintenance manual for filling the system.  I practiced this procedure a few
times while flushing the system with fresh water, prior to filling with
coolant. The practice was so I wouldn't waste too much coolant when refilling
with the real stuff.  Just in case your rig doesn't have a decal, I've copied
it below.  My comments are in parenthesis.
Steps 5 thru 8 are best performed with a helper. If you have a helper, you
will not have to waste coolant.  I had my son turn on the heater loop inlet
gate valve from inside the bedroom, while I caught the antifreeze being
purged from the heater loop outlet petcock under the coach near the frame
rail at the left rear.  Meanwhile, the petcock alongside the surge tank was
cracked open, but I had a hose attached to it, and caught the coolant purging
from it in a jug for reuse.
The book says not to let more than 1-1/2 quarts escape either petcock.  This
would be very difficult without a helper to open the gate valve. At least,
you'd have to be 10 times faster than I to open the valve from inside the
bedroom, then run thru the coach, then around to the back, crawl under, and
turn off the petcock.  You could easily waste a gallon or more from each
petcock, and probably create a big air bubble in the system.
When our rig was new, I had to add coolant after each trip for the first 6 or
8 outings.  It's probably because of the air space in the surge tank above
the filler neck.  The air must displace between one and two gallons of
coolant.  The air is eventually purged through the overflow bottle and replaced