We have a Carriage Carri-Lite 2007 model with the same type of holding tank cover. Ours came with an area cut out that was supposed to allow access to the valves (ours are Drainmaster 12 VDC electric valves) but the access port was really just too small, so I enlarged it. It is a pain to drop the entire pan, and it's even more of a pain to re-install it, given there's no real structure to the pan. So after having dropped it 2 or 3 times over the years, I decided to throw it away, and I used thin angle steel to frame up a new cover and used sheets of about 1/2" think polypropylene from Lowe's to screw on to the framing. I also created a larger access port that can come off with six screws for fast easy access. I expect for you it will be easiest to drop the whole pan and then cut out a suitable sized access area. Working it up so that the piece you cut out can be used as the new cover will be a bit of a hassle. The big picture issue here is that Carriage, like all RV manufacturers, virtually always do things the fastest easiest least expensive way they can that will work for a few years. This was confirmed in my mind again this past winter when I totally removed the kitchen slide, removed the original flooring in the main area and had new flooring installed. I had to replace the slide roof membrane and ended up having to replace the rollers the slide rolls on and make a few other 'upgrades' to things that were not engineered to last a long time in the first place. So, all that to say sorry Vince, there are no easy solutions. Perhaps I should say the solution could be described as 'easy', but it you will find the process rather frustrating! If you do drop the entire pan, you may want to consider installing the electric dump valves. You can find them at www.drainmaster.com
. I've interacted with them several times and produced a couple of videos for them, and they are easy to talk to and give good service.