The way my trailer was built, and I suspect yours is identical, the black tank is in an area that limits putting a larger tank in. My grey tank usually fills up before the black tank is even half full. Black tank size has never been an issue for me. I did change the 4'' drop
axle for a straight
axle. The parts cost about $200 and I did all the work myself except for a little welding. There are many ways to go about this modification. New springs, shackles,
brakes might also be needed. Wheels and
tires could be between $300 to well over $1000 depending on personal choice. The
axle lift is a great mod in my opinion and many have already been change. If you are looking at a trailer, it may have been done already.