IMO, leaving the tow vehicle hooked up during setup will not hurt anything, and should help reduce the drain on the batteries during setup. Are the batteries new??
I have had a group 31 battery on my 2005 Jay Feather LGT for about 6 seasons now. It provides about 30% more reserve than a group 24. When my new 2013 Eagle 28 BHS comes in, I told the dealer I want the same size battery on it too.
FAA A&P mechanic, ex-RV dealer parts dept
Retired B-52 Gunner/Flight Examiner, B-52 Crew Chief
2016 Jayco Eagle HT 29.5 FBDS w/ 15,000 BTU AC, 50 amp service, upgraded counter top, thermal pane windows, elec. stab jacks, outside grill, backup camera and MORryde pin box.
2011 GMC 2500HD DURAMAX/ ALLISON