I just did this a couple of weeks ago and my dad did the cutting with his skil saw (he does not trust me with his saws...).
A few years ago, I made a shelving system out of PVC pipe and plywood, press fit against the sides of the closet (so nothing is screwed into it) and when I did, I made it so that the PVC frame goes all the way down to the floor. Then I put a shelf on top of the wheel well, so as not to lose that space, but the only way to get to it was through the first shelf of the closet opening. I was only doing week-long trips at that point, so I didn't really need the space down there. But since I was planning this long haul to Nova Scotia from Minnesota, I needed it.
So we made the hole just under the reinforced fiberglass at the base of the existing closet opening and down to the top of the wheel well. I need to get some edging for it (didn't happen before I left), but otherwise, it's perfect. We were making new cupboard doors already, so I made a new taller closet door that covered the new opening as well. Unfortunately, the wood we used was warped, so it doesn't even pretend to close anymore, so I'll have to redo it when I get home. A bungee cord keeps it closed in transit.