i used to be in the caravan business and these are the same batteries i used might be worth looking on the net at camping shops or ebay i used to fit these little guages in camper conversions but the best one ive had is a proper battery discharge tester metal box with dial on it and 2 croc clips, conect to battery and it will tell you if its good to go, you can also do a discharge test to check the cells are ok and if you connect it to your charger it will tell you if thats up to the job,
Cost about £30.00 screwfix