Dipodomys, I was faced with the same dilemma as you a few months ago when I was thinking of replacing the two Victron 60AH GELs with a single 200AH LifePo4 and I measured up as you did, but realised that, whilst it would fit in the space lengthways, it would not go in through the door diagonally. This was a disappointment and I decided to leave it that stage. Not only because of the size, but also because of the cost.
Since then, I have done a bit more research and realised that whilst, it would be nice to have the larger battery, the 160 watts of solar would probably not be sufficient to fully charge the bigger battery - especially taking into account the British weather! So I have decided to go ahead with a 100AR Lithium at a cost of about £300 ($370) which is a good price in this country. It will give me better capacity than the other two batteries, bearing in mind it will comfortably discharge to around 5% of it's full capacity and will charge probably twice as quickly.
My problem is that because I have changed to LifePo4, I will now also have to change the shore power battery charger as it is not rigged up for lithium (unless I change the the small IQ4 Smart Charge controller for the IQ4 LIFEPO smart charge controller and I can't find one of those in the UK!!)
I will also need to change the Blue sea VSR which links the vehicle alternator to the battery in the camper, for a Victron Orion Dc to DC charger, as the Blue Sea is not able to adequately control the current coming into a lithium battery from the vehicle alternator.
I have also decided to change the Overland Solar MPPT controller for the Victron bluetooth version as it is a lot more controllable.
As well as all that, I am also going to fit a Victron inverter so that I can actually use the mains socket when not connected to shore power. Its going to be an expensive exercise, especially as it will be all Victron equipment.
Anyway, what I'm trying to say is that, to fit all these new items in, I have had to take the front of the cabinets off. I had already taken the upper front one off to make a lot of mods to the water system - new pressure vessel to stop the pump cycling, 0.1 micron filter to make the water taste great and take out all the nasty viruses and some additional pipework so that I can take water from, "clean" sources and pump directly into the tank. I have also included the ability to fill the water tank from the mains via a hose through the city supply inlet.
The top came off relatively easily, but the lower one was a pain and there were lots of screws to remove. Veryactivelife is correct regarding cutting through the, "silicon" although I was worried about using a utility knife and used a small pick with a 90 deg bend on the end which did the job perfectly.
The other thing I noticed, which may be an issue for you (depending on your model of camper) is the fact that the gas pipe for the hob comes in to the battery box at the base and does a right angle turn up the inside, but next to the front, if you see what I mean. With it being in this position, it meant that I lost about 1 1/2" to 1 3/4" on the length of the battery box.
If you are taking the lower front off, all I would say is, don't force anything or you might split the plywood and make sure that all the screws are out. For info I have a Wildcat camper (German version of what I think is the Fleet in the States) with a front dinette.
I have plenty of photos if it would help.