So, with the Corian and plywood out of the way I started cutting with an angle grinder. I made a couple of mistakes - which is to be expected since this is completely blind work, but if any one else tries this it will help to know what is actually under there.
The first mistake is there is a cross bar visible behind the rear cut - it is welded to the step. I didn't know it was there so my initial rear cut was actually behind it. On the right you can see where my side cut extended back to meet the rear cut. It just happened that the cross bar ended just short of my side cut or I would have noticed it. I used a reciprocal saw to finish the cuts, and it slid past the cross bar or I would have noticed it. The really bad news is that there is a hydraulic hose right below and behind the cross bar, and I cut it!
With the panel out I now have access to the top of the manifold. In future problems with the valves will be much easier to resolve, and checking and adding hydraulic fluid will be much easier. Still not trivial to reach some of the components, but it will reduce the number of times I have to crawl under the coach to sit in the generator cavity and try and work from the side.
Next up I need to do some electrical testing to find out why the pump is not coming on. I may just put 12V to its inputs and see if it turns. My suspects are teh pump motor, the relay, the HWH control box, and the multiplex system. The HWH control box is showing a yellow LED when I activate a slide switch, but it should also show a red LED. HWH does not answer user inquiries anymore - they used to be wonderful now they are downright terrible - so getting basic info like what the pin positions are on the plugs going in and out of the box is not possible.