I always install a blade down the external piece of pipe to prevent this from happening.
You can try a couple things... I fixed mine on the Heartland by doing the following.
Open under belly below the tank are, don't bother cutting holes, you need to expose everything.
Find the valves and remove, clean a and replace rubbers.
The valves work best when the cable sheath is straight which may not be possible but really long large radius bends seem ok as well. Also, secure the sheath so it can't move.
In my heart land I installed the valves in it the pull rod facing the opposite side of the trailer than the pull handles and mad a really big U turn using all the sheath and the widest radius I could. I also fastened the cable to frame rail and a piece of angle I bolted to the frame to provide vent the sheath from. Moving.
Your configuration may be different but my solution worked. My bro in law converted his to regular valves with a little plumbing reconfigure and longer rods so the valves are still in the heated area... If you don't plan to winter camp and donned need the valves to stay thawed, reconfigure your plumbing and install blades under the camper....
And, there is always electric valves