If you have a 2017
Casita 17 footer, you have plenty of room for 15" tires/wheels. I'm surprised you don't have 15" wheels already, but they are now standard on later model Casitas.
All you need to know to get the right wheels, (rims,) is:
15" diameter X 6" wide.
"Zero" Offset, (which means the rim is equally divided on both sides of the lug nut holes, inside to outside.)
The lug nut pattern is
5 Lug nuts on 4.5" centers.
(And fwiw, the lug nuts on the standard Dexter #10
axle, (which are what
Casita uses,) are
1/2" dia. X 20 threads per inch.
If you decide to go with aluminum wheels, be sure to get the right lug nuts for them, because the steel rims use lug nuts with a different "shoulder" than the aluminum wheels use. And be sure that you ask for
metal valve stems for the rims, and also be sure to
have the tires balanced. This is important!
And as to tires, I recently upgraded, (last year,) to Good Year Endurance 225 X 15 ST, Load Range E tires, and I've been very happy with them.
And as to adding a bike rack, they do make ones that can be mounted on the front of the trailer, (over the
propane tanks,) or ones that you can insert into a custom made 2" frame mounted rear receiver hitch,
but be forewarned that very few bike rack manufacturers will certify their mounts for use on the rear of travel trailers, and the few that do exist are several hundred dollars. Caveat Emptor.