Most likely the relay would be in with the other chassis fuses and relays, often a box in engine compartment or perhaps under the dash board, according to where the RV builder arranged the wiring loom.
The low pressure switch generally just passed 12 v through if the pressure is proper, yours might also be used to control other items, like the fans on condenser coil and the like. If pressure is too low, the switch would have 12 v on one wire and none on one or more of the others.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI