Great idea to have multiple ways to fill the tanks.....I have a deck fill and an access hatch on mine I can use. I need to get into mine and do a good cleaning, but the drawback is that the old tank is a molded in tank (65 gallons, which is great for a small boat like mine) under the vee-berth and the lid section is fastened with a million screws and 5200 so I'll have to take a saw to it to get it out and I'm not sure how to fasten it back down so it doesn't leak afterward. That will be a project for next summer.
I did change out the holding tank and hoses on my boat a while back. It took about a month of working a couple of hours on Saturdays and Sundays to get it done. Before I started the project I thought "OK, I'll be conservative and allow an entire Saturday afternoon for this......" But, I am happy with the way it turned out. I have the y-valve locked in the pumpout position because I haven't installed the hand pump for overboard discharge yet, but how long could that take?