Suburban or Atwood.......
BOTH heating sources can be ON at same time. Doesn't hurt anything....faster recovery time.
As for your Suburban.
They use 2 different sets of t-stats.
A 120V AC set for electric
A 12V DC set for propane.
Both should have same pre-set temp settings (normal 130*F and high temp 170*F)
Electric should be just as hot overnight as propane.
Check wire connections on t-stat (left side electric with black wires) and t-stat tight. Dirty/loose connections can affect operation.
Optional 120*F and 140*F t-stats available BUT 120*F hard to find any more.