Most all the adult camps as mentioned don't cater to those who want and "extreme" experience..........I've been to Newks(go regularly), Saddlebrook, Sanchez, Van De Meer, Club Med, Vic Braden, Vilas Academy in BA and a few in France.
Honestly Saddlebrook tries to really work you out but it also depends on who else is there but it is the most intense along with - Club Med (Sandpiper and in Pompadour, Fr) have the "intensive" tennis and is very very good BUT the extra cost can put it up on the price scale after you pay for the xtra.
One thing to look at is the LEGENDS week at Newks.....yes it's pricey but along with the ability to play and party (lots and lots and lots of party at night!) the tennis is for Newks a real tough week. Most guests are solid 4.5 and up easy and the workouts and tennis tough (but fun) - I've done it about 12 times in the last 25yrs and its the hardest week - and if you are going to compare flying from the states to Europe or S America the cost would be about the same.