2004 winnebago adventurer

2004 winn adventurer
workhorse chassis 8.1 vortex
when under 30 degrees transmission wont shift starts out in 2nd gear i believe. if i drive a mile or 2 shut it off and restart it shifts fine. this always does it when cold,then i can drive all the way to florida with no problems. ive had rv for 8 years .but im leaving later this winter when its sub zero. im afraid it wont warm up enough.
any clues thankyou