2008 paccar px8 in 335 peterbilt

hello from Alaska yall .so I got a 2008 pete 335 after I noticed combustion in the coolant system . my business partner sent it to the dealer before I could dive into it along with do my research. its been there a day now and they are saying its got either a bad head gasket or bad head or maybe a bad block . I asked them what test they preformed and was responded with "we have confirmed it is combustion in the coolant system and that its almost right after engine start up it begins to get pressure, we hooked a carbon tester to the surge thank and it should be a head related issue . " bare in mind I'm a heavy equipment and truck mechanic/ driver and have most tools they do besides the computers and can run most test they can mechanically . anyways ive been reading and so far it seams that there is 6 possibilities that could cause combustion pressure in the coolant and the dealer hasn't checked any of them that could fuel it . from what ive read in the last 6 hrs on that engine the primary possibilities are , 1. head gasket , 2. cracked head , 3.bad piston sleeve, 4. bad injector seals , 5. egr leak and number 6. turbo because the egr and turbo are water cooled . and could be leaking into the coolant system . my question is why wouldn't the dealer do a cylinder compression test to see if it shows a head/gasket problem along with check the turbo , injectors, and egr system before saying I have a head related issue or a possibility of needing a engine overhaul... I'm just on here wondering what yall think and would recommend ?