Tow Pro: caravan tyres and rims

I own a tandem 3 tonne simplicity suspension system on a roadstar caravan and would like to increase the load capacity and height. of the caravan. Which would be better just to increase the tyre size to 205/R14 LT 109 load capacity 684mm overall diameter or increase the rim size to 15 '' and increase the tyre size to 225/70/R15 696mm overall diameter as both tyres would be getting closer when installed ? What would be the minimum distance between the tyres? Thank you .

Peter, VIC