A 10/2/50 charger probably means 10 amp high rate, 2 amp low rate, 50 amp for starting. It'll have a duty cycle limit for the 50 amp setting though, and should be anticipated that it might hit a thermal cut-out or even be damaged if left running at the 50 amp rate for a long period of time.
I didn't see what bus you have, but if it's a full size and runs a pair of 8D batteries as many do, you may want a higher-powered charger. I'd have to make lots of if's and guesses in the math, but I can say I've had times when I used my 8 amp charger for a pair of well-discharged 8D batteries. It took over 24 hours. I have another charger that goes up to 40 amps, but in this case I was in no hurry and my little one is convenient and also better-suited to being left out in case of inclement weather.
The batteries in my bus are aged and in poor condition. If it sits for several weeks unused the batteries will be too low to start. If yours sat for a year with the batteries connected, they may have been deeply discharged and their capacity is permanently reduced.