For my propane tanks, I chained the two tanks together and to the TT frame using chain and padlocks I already had, so $0.
Haven't done anything about the batteries. If an evil doer sees the chain around the frame for the tanks, hopefully he will believe the batteries are secured, too, and look for something easier.
Sherm & Terry w/rescue Eydie (min Schnauzer) & Charley (std Poodle)
SOLD:2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18, 365+ '20
SOLD: 2006 Ford F350 PSD, 4WD, CC, LB, SRW, Camper pkg., 375,000mi
Full timing: Some will think you're crazy, some will be envious, just enjoy the freedom!