Fresh Water Tank Access

[Badco8]

My 76 AS Sovereign leaks around the front edge of a large metal box framed square section in front of the wheels. It hangs slightly lower. Being a new first owner of an older AS, I am assuming that this is from my fresh water tank. Is there an overflow or vent and is it leaking due to too much water or possibly leaking thru the small holes that some models have for heat from a blower pipe from the furnace? Is there an access door in the floor inside?
Please where do I start. The whole plumbing layout is a mystery to me.
I guess that being an older tank, I should consider total replacement and also to the grey water and black water tank.
Please!!!!!!!!
Some good logical advice and links to helpful threads with drawings & explanations.

[LI Pets]

You have two hoses, one is the fill which goes into the top, it is a 90L fitting under the sink-stove, near it is a 1/4" maybe 3/8" vent hose that may run next to the fill hose.

Next there's one hose from the water pump that goes down by your heater vent hose under and to the middle of the tank at the bottom.

Crawl under your trailer, if you look under that large metal box holding the water tank there is a 12" X 12" plate with a bolt in the center, you may need to use some heat to get it lose, but if you use too much you'll ruin the tank.
When that is removed you can see the other end of the hose to the water pump, and all the wires which go the gauges telling how full/empty it is.

If you had a pic it would help.

[Badco8]

Thanks so much Bob. Your info has been a great help. I am comprising folders of information from the forum and people like yourself as I love this AS and love projects ( This is surely one ). It will always "outshine" the vinyl and boxey ones surrounding me at our private campground.
Again thanks.