You are going to need more to finish your upgrade
According to your profile you have a 1996 vintage RV
Atwood changed controls schemes on their combo water heaters in 2004
One set of t-stat for both electric and propane, circuit board that controls electric and propane and DC circuit to accomplish this
So you will need 120V AC circuit breaker (twin CBs are easy way to accomplish an individual feed to electric element)
A dual DC on/off switch panel ---one DC on/off for electric and one DC on/off for propane (uses the SAME DC feeder FUSED circuit)
Here is a manual explains wiring and a wiring schematic for new version of Atwood Combo Water Heater
Manual.......see pgs 23,24,25
Combo DC On/Off Panel & wiring
Panel $9
