Tank capacities on my 29rbs15

[Choward]

This is a lite 5th wheel with that model number on the title,

I can’t find any specs in the owners manual or online. It seems to be empty a lot, and while transferring a 55 barrel of water into it after running out, i couldn’t get the whole barrel in it

Just wondering what it is and can I replace it with a larger?

Thanks,

Chad