Hi Gambler,
Two things I would try, Firstly, go to 12N24 ride on batteries, they will have much more reserve current capability to get up steep slopes and may be enough to compensate for the power drain under load. If it still struggles, replace the wheelchair motors with windscreen wiper motors. Very early motorised wheelchairs used them before purpose built motors became available. They have a worm drive reduction gear built in so could probably be used as direct drive to the tracks. The worm drives also have a natural brake ability when powered off and could provide a bit of added safety on those slopes.
