Checking the Monaco Brochure for a 2000 Exec*, the OEM tire size is 295/75R/22.5.
The 255/70s you have are way lower in load capacity and their smaller radius may have been affecting Speedo and ODO readings. Using the Toyo tire book as an example, their M122 in 255/70R22.5 have a Max of 5510#/5070# @ 120psi (Single/Dual) while the OEM 295/75 size rating is 6175#/5675# @ 120psi. The Toyo book shows M144 in 295/80R22.5 size at 7830#/6940#; the book does not show a 295/75 size for the M144.
While different tire brands may vary, you now have tires with substantially lower max capacity. Unless some suspension-clearance modification has been made to your coach, the 295/75R22.5 tires should fit and restore you to the OEM carrying capacity. If you want to go to 295/80R22.5 tires, you can check the size vs the OEM 295/75R22.5 to see if clearance may be an issue there.