I went through the same thing with my Hurricane right after I bought the coach. The heat exchanger tubes were leaking where they were swagged into the tank. The color of your leaking fluid would give cause for concern though as the Hurricane fluid should be clear and pink or green depending on the antifreeze used. It sounds like you may have both a FW and coolant leak and the crud is stuff accumulated at the bottom of the HW heater. When you pull the coolant lines you may not see the "crud". If you do, the system will need to be flushed.
Fortunately, this is an easy fix once you get the hang of it. I can now R@R the Seaward in 15 minutes although the first time was closer to 45 minutes.
The heat exchanger is used by the Hurricane system to heat the hot water. The two freestanding heat exchangers are on the HW cold water supply and are used to preheat the feed water to the HW heater. There is no direct connection to the engine coolant at this point. When you pull the HW heater, you will lose all the Hurricane fluid which is not a problem. However, the recommended coolant is a propylene glycol that a bit harder to come by so you should get that in advance.
A couple of notes:
When you pull the HW heater, it's held down by 2 U brackets over "ears" on the back and front. Remove the front one and just pull the heater out and it will slide out of the back bracket. Then just slide it back in when installing.
Recharging the coolant is a LOT easier than the manual states. I chatted with a tech at the factory and just ended up filling the resovoir, leave the cap off, and manually ran the circulating pump. Keep adding fluid as necessary. I found it useful to run the pump on for 15 sec and off for 15 sec. You can hear it when it's pumping well. Also pump the line by squeezing the pump discharge hose. After it is done, start the Hurricane and let it run for a couple of hours or so periodically and keep checking the fluid.
