Welcome to the forum...
Don't know much about the '96, but, Cummins and Allison are a good combination.
I would certainly think a close inspection, by yourself, and perhaps a qualified mechanic, and or RV tech would be in order.
Pay attention to the smell when you first go inside...does it smell even of the faintest of mold-musty?
That would indicate a leaker...are there any indications of delamination on the sides...you get the idea. Look for obvious signs of impending disaster, and seek the opinion of someone who really can have a good look, and make an educated statement.
Keep us informed...and..whatever you end up with...have fun!!!
Back to a 5'er after 4 yrs fulltime.
Pulling with a '14 2500 ram, 6.4 Hemi.
Montana High Country 31RE