Answering your question.
Check that the batteries are not overly swollen, a poor battery tends to have bulging sides.
Check each battery voltage when charged they should all be about the same ie 13+ volts depending on how long since they were charged, if there is a big difference in battery voltage that could be an issue.
When driven for say 30 mins, re check the battery voltages, if any are obviously low and well below 12v at rest they could be poor.
Check the electrolyte level, some later Trojans have a small dot and If I recall that should be showing white not black.
Look for any exceptional corrosion on the battery terminals. Also tighness.
If you have access to the kit and software for the Delta Q charger , you can see how many times its been charged, if any charges were aborted before completion. It gives an idea of how its been charged. Ie its only got 300hrs on the clock, but its had to be charged after every half hour to get there.
Get some form of guarantee from the seller.
If a vehicle is regularly used , ie a couple of hours each day, then my customers have been seeing about 1.5 to 2 years life. Others who use them a few hrs a week are seeing many years of use.
Lead acid seem to degenerate after about 500 cycles of charge discharge, if properly maintained. That also fits with my users experience when used daily.
Under the seat towards the centre back, is a round bodied contactor with two 5/16 posts. On this is a short harness connecting to a multiway plug, check its not burnt inside. In the same area is a small block of 3 fuses, on the end of a lead, may still be in a plastic cover, check the centre one isnt melty. Both thes issues are associated with the charger and high currents causing failures over time. Not a big deal but both can be a show stopper.
Check the CV and steering gaiters, as well as the suspension bushes.