Can I put bigger tires ?

Suspect any improvement will be marginal at best. Pretty expensive change considering the cost of tires, rims and simulators. You might be looking at several thousand to make the change. There's also the question of wheel well clearance to consider in particular the fronts when you are making hard turns.

There are a few gas powered chassis with 22.5 rolling gear. Ford started offering the size on the heavier F59 chassis a few years ago and I'm pretty sure Workhorse was doing the same when they were in operation.