The answer came courtesy of Airstream:
Originally Posted by airstream
The wheel is a 14 x 6 on 4.5 centers, 5 lug with no offset. I hope this helps.
So, if you want to upgrade the tires on your 16' Bambi from 215/75R14 LR Cs to 225/75R15 LR Ds without upgrading your axle you need a 15" wheel that matches the pattern above and has sufficient PSI and rim weight ratings.
I found one in aluminum at Eastern Marine - for details see below.
thanks and enjoy,
Utility & Boat Trailer Tires / Aluminum, Galvanized & Painted Trailer Wheels
15" 5 on 4.5 Lug Aluminum Trailer Wheel
STOCK #: 1350116
Rim Pattern: Modular
Tire Diameter: 15"
Rim Width: 6"
Rim Finish: Polished Aluminum
Rim Weight Rating: 2150 lbs. @ 65 psi
5 lug on 4.5" Bolt Circle, 3-1/8" Center Hole