I just noticed that this 4 year-old thread I started has been resurrected! I promised to report on how I fared trying to remove the frozen deck cap for my holding tank
and I did not keep my word then , so I will do so now.
Well I succeeded in removing the cap, without destroying it, and have been thoroughly enjoying the experience of pumping out my holding tank
Here's what I did: I have a friend who works on machinery and swore that an impact drill will free up anything. He very kindly fabricated for me a customized bit with two steel pins welded to a steel bar. The pins fit into the two holes in the cap and he welded a nut to back back of the bar which fit a socket bit that I fitted into a 110 V impact drill rented from a tool shop. He was right! The intense vibration from the impact driver broke the cap loose!