Trans/AIR normally locates their relay panel in a box under the bus... on my red bus its actually oddly out in the open.. and yeah things get wet and need rep[laced on it..
theres no thermostat switch on thew Trans/AIR units.. the condenser operates anything the compressor engages.. on a van cutaway that can be when the rear unit is off if this is a Tie-in system (90% are).. so if you are using only the front A/C, front Defrost, front Bilevel your compressor can come on.. (thats controlled by the original Van selector switch (looks like a car A/C)..)
the condenser fan should never run when the bus is turned off so its likely as you think, a stuck relay..
id look for the panel in the battery box, sometimes in a box on the back deck.. ive found them under the bus.. follow the wiring harness from the under-body condenser it should lead you there to it...
Trans/.AIR is still in business and will have that whole A/C system logged in their systems by your bus VIN.. and may even have service documents for you if oyu cntact them. I have bought parts a couple times from them for mine..