I used my L185DT with a 5' bush hog for years. At times I was in low gear to keep the engine running, but it always managed. The closer to the ground, the more hp it takes. You should not have a serious problem with a 48" tiller. I had made a dualie arrangement for mine, and had plenty of traction. I never pulled a harrow. I don't believe it would be a problem, but again, the wider the implement, the slower you will have to go. I'd think you should get by in 3rd or 4th, but probably not 5th or 6th.
Frankly, I loved my 185. GREAT tractor!!