Let's dispel some things right off.

1- "you can't put 15 inch rims on your Casita unless you have a hi-lift axel"

2- "changing to 15 inch rims doesn't get you anymore clearance"

This animation is a solids model I put together some years back so that I could determine the max tire diameter that I could get in my Casita with a Hi-lift axel.

There is a big fat tire in the animation model shown but further investigation below will show what sizes you can actually fit in your egg.

The main thing to notice is the green dog bone. The Dexter Torflex axel on the casita is a rubber-twisting affair that applies torsional resistance to the wheel's movement about an arc. The center of that arc being the axel placed some distance in front of the wheel spindle dog bone is the part that connects the axel to the the wheel spindle.

Looking form the outside in, the axle centerline is on the left end of the dog bone attached to the frame. The right end has the spindle mount. So the torsion revolves around the centerpoint of the axel, not the spindle.

The red pieces are the frame. The frame is exactly the same no matter which axel you choose, standard or high lift. What makes an axel a high lift axel is simply the starting angle of the dog bone.

This is the first of a series of images that show what your tire clearance would be with zero degrees, 10 degree, 20 degree, and 30 degrees axle positions... driver side (left side) shown. There are six tire diameters shown too: 24" to 30". The image (above) is what my high lift axel looks like when the egg is jacked up. The dog bone is flat or at "zero degrees". Here is an actual photo (below) of my axel on jacks... passenger side, right side.

Thus enters the sum of the confusion and big chance to understand it all at once! Casita uses two different axles. Standard (regular) and high lift. It is a difference of dog bone degree not height. The stock Casta axel is a 10 degree UP axel. The high lift is zero degrees or neutral

There is this much clearance, fore and aft, for my big off-road bias ply tires that have a 30" diameter. Further, some pretty heavy rock road jaunts showed no signs of rubbing. If I ever have these mounted again and hit a big hole at highway speed I will immediately stop can get out and check for rubbing.

The close-up image of the standard axel tire diameter clearances show you can actually put a 30" diameter tire on a standard axel, right? Well who knows really? I have assumed a load and driving conditions that dont flex the standard axel passed 30 degrees. Maybe you will flex it more and rub. I also did not take into consideration that the axel could have been welded up to a half inch forward (bad) or a half inch rearward (good)... and take note that the sine of 20 vs sine of 30 makes a BIG difference in moving the tire closer to the front of the wheel with the standard axel. Add to this that my measurements to build the model could easily be off 1" in all directions. A good way to find out the clearance you actually have is to place a large glob of clay inside the wheel well and measure after it has been milled off by the tire (make sure you measure over a big bump and have the max load in your egg).

It can be seen rather clearly that larger tires mounted on a 15 inch rim do not automatically hit the wheel well and thus dispels the "you can't put 15 inch rims on your Casita unless you have a hi-lift axel" comment. Of course you can if you check the diameter of the tire that goes ON the 15 inch rim. The comment, I think, comes from the automatic opinion that fifteen inch rims dictate a really huge tire diameter and they do not. With a fifteen inch rim you have twenty times more tire picks than you do with 14" rims.

NOTE: 14" Carlisle is right around 26" in diameter and a 15" will be 28". That is  one inch lift with a C vs D 1800 vs 2500.

At the same time it dispels, "changing to 15" rims doesn't get you anymore clearance". I have no idea how to reply to this comment other than to say, "look at the pictures"...