I don't believe there is ANY relationship between LPG and 120VAC. You select Either Neither or Both and it does what it can do. Including running both till one t'stat is satisfied, then the other.
It seems from what you say that the LPG thermostat is set (translation: Built - Don't think they're adjustable) higher than your electric t'stat. AFIK the "standing pilot" LPG heaters use an adjustable t'stat but the DSI types do not. Also, none of the OEM electric t'stats seem to be adjustable. If you want LPG "backup" for your 120VAC operation I think you'll have to turn it on yourself.
Our coach is 30A, and we often find we can't run our 15K A/C and the WH on Electric in the summer. I've found that the DSI LPG feature provides hot water so quickly, that not having a full hot tank from 120VAC operation isn't really a problem. I can turn the WH on partway through a meal and have plenty of HOT water for dishes. 10-minutes or so.
