Probably has a round silver solenoid that is used as the boost and house battery charge relay. Very common for these to go bad. Sounds like yours have welded.
Charles
2007 Winnebago View 523H on a 2006 Dodge (Daimler-Chrysler aka Mercedes) Sprinter 3500 chassis. Bought Sept 2015 with 18K miles on it, Prog Ind HW30C, Prog Dymanics PD4645, Chill Grille, PML/Yourcovers.com deep alum trans pan, AutoMeter 8558 trans temp gauge, Roadmaster sway bar, Fantastic Ultra Breeze hood, added OEM parabolic mirrors and RH aspherical mirror.