I've done an RIS with 42 pounds of malt(and a pound of molasses)and 11 gallons of water in my converted keg . The only issue is sparging, it takes forever because the grain bed will compact considerably. I prefer fly sparging on a grain bill this big instead of doing 5 or six "batches" required to collect 13 or 14 gallons of wort. Efficiency sucks on a grain bill this big but it can be done.
The last time I brewed this beer I used my old ZapPap mash tun for about 10 pounds of the malt and boiled it in a smaller kettle. I collected a total of about 17 gallons before the refractometer indicated the runoff was around 1.010, and it took almost 3 hours to boil it back to the 11 gallons I wanted.
But it was worth it. I finished the second keg of that batch almost 3 years after I brewed it and it was still an amzing beer. I had to keep telling my friends the keg was empty or they would have finished it in no time.