The radio itself has equal output to all channels by design. Given the others with the same problem...lets assume that it is not the fault of the radio that the sound level outdoors is not sufficient. I'd suggest that :
1. The speakers suck anyway and getting highly efficient (look for a high SPL number in the specs) new pair of some quality...will both improve the loudness AND the quality...but maybe not enough to make you happy.
2. Outside you are dealing with a large unenclosed space rather than an enclosed coach. This takes POWER to overcome. To keep it simple...I suggest the addition of a small car stereo 2 channel amp that will take high level speaker inputs.
Like this: PYLE PLMRA120 Waterproof Marine Amp (240-Watt, 2-Channel) - SHOP.COM
You could wire this up and then adjust the AMP volume outside to match a comfortable sound level from the radio inside...and the radio volume control would govern both...while still giving you the opportunity to listen to either separately.
If you're not comfortable with the install...any Best Buy or Auto Sound dealer should be able to do it easily.
You'll need decent speakers to accomodate the louder sound level anyway...so I'd try step #1 first and then go from there. Get CIRCULAR (not oval) speakers in the largest diameter you can fit...6.5" would be great.... but do what you have to to make it fit...we ain't talking hi fidelity here just higher volume and less distortion when you do that!