Just on the Edge of Possible
Welcome aboard the RoadTrip America forum.
By the most direct route possible (I-90/I-80/I-76/I-70/I-15) Boston to San Diego is roughly 3060 miles. By your proposed I-81/I-40/I-15 route, it is about 3150, depending on how, exactly, you get to I-81. In either case, you'll have to do over 600 miles a day to make it in 5 days or over 500 miles a day to make it in 6. You can drive 600 miles in a day, but it is work and not an 8 hour joy ride. It will get old after a few days and you and your friend will still be cooped up in a very small space for more days to come. 500 miles a day will allow you some time out of the car, not really all that much - but it will be a big help. When I moved to Tucson from the DC area, it took 4 full days of solid driving and you are talking about adding a full day's drive to either end. If I were you, I'd treat this like the hard job that it's going to be and just try to make some steady progress across the country. If after 4 or 5 days you find yourself a bit ahead of schedule, all the better. Then you can look for some place among the wonders of the west or southwest to spend your 'free time'. But I really wouldn't count that chicken until it has hatched. If you're going to be driving a truck, pulling a trailer, or even just driving a fully loaded car, things are only going to go slower. Sorry to throw a damper on your enthusiasm, but it really will be better for you to plan on not having any spare time and then be pleasantly surprised (an outside possibility) rather than expect this to be a sightseeing jaunt and finding yourselves with only a day or two left to get to San Diego when it is still far away (a very real possibility.)
AZBuck