These things are designed to be water tight, the lid unscrews easily for access. Yes this came up again in a discussion on RV.net last night. Fella had a fill hose adapter blow off into his tank-"How do I get it out. or does it matter?" Consensus was, Well since the hose is inert shouldnt hurt to stay in there but the "if you had an access port..." came up. One guy says he'd like to put one in to check his level. Then another fella mentioned he mostly used his port when using jugs to fill the tank.

DOHP! in the all the years I've thought of this project, camping & making 20 trips with a gallon jug, holding a funnel trying to refill the tank -just never occurred to me. Ya know if we had a tank port we could just dump it in. Could also use the Britta or similar to filter the water before dumping.

Any way I looked on line, deck lids range (for the same brand $10-30) then I saw you could get them with clear lids. Hmm that would be kinda nice. Anyway on my home tonight I stopped by the local RV/Marine shop. I said I was kinda thinking of adding an access port to my water tank, folks are using "deck lids". He walks over to the shelf. " this is all I have, just came in, a 6" deck lid with a clear lens. $16". I said sold. Went to the hardware, picked up (12) 1" 8-32 phillips CS Stainless screws 5/8x #8 fender washer & nylok nuts, and tube of silicon. I Highly recommend getting socket head countersunk screws, I didn't only spun the head on 2....

I modified the ring, adding 6 more holes. This helps insure it doesn't leak or warp and give more support to the tank top.

Ok-Next making a tool to fit the lid & modify the tanks step board for easy access.

To insure alignment I placed the 2 small blocks in the lid recess, applied wood glue, put the cross piece (handle ) and lightly clamped. Removed after couple of hours and used 3 nails each side. Sanded and sprayed some clear on . Also drilled for lanyard to tie off in water tank area.

Here you can see the profile I needed to match.

Lastly modify the box top for getting access to the new water tank access port

Nice would be a simple piano hinge. But part of the board is under slide, so id need to cut-hmm I just re carpeted, there's no structure to the box (it relies on the top being screwed down for integrity) and there's no room to add anything to support the hinge side or reinforce the outer box wall to the camper wall. He hawed around for a while then decided all I needed was to duplicate the original top, put a false top on the box , then it wouldn't need the carpeted board for integrity. I could make the top board just slide in. I used a piece of plastic cause that's what I had laying in the shop. Works well but Ill probably at some point add a piece of flat bar with cross flanges welded so I can secure the outer box wall back to the camper wall. Piece of 1/8-1/4 ply would alos work for the false top.

I used 2 simple hook & eyes to hold the board front on.

I also added 2 small angles at back edge of board to insure it doesn't tilt up. Board slides in pretty tight. Because Ill bre pouring thru 2 holes Im going to find a plastic bowl that will reach into the tank, cut the bottom out & install screen. Or a large funnel. It'll act as a splash guard when refilling with jugs (juggin) and catch any large objects that are not supposed to be submerged in our drinking water.

Update-

Couldnt find a funnel or bowl that was short and also big enough around, so made one. Im sure theres somthing out there. Used some scrap poly sheet I had. Formed approx 11 1/2 od X 3 id X 5" cone. Siliconed the lap and used #8 x 1/2"screws& nuts. I was going to get some screen and happed to see this old tube bumper cap on the bench. Wow-perfect fit, just plain luck, got a lip so cant push thru, extends the reach. Drilled it full of holes. If the water chokes Ill completly remove center of cap and put coarse screen on it. It just slips in didnt seal

As Ill be pouring thru 2 holes when using jugs-this will work great. Screen (Cap) also extends the reach into tank.

Final note: Another reason for adding the tank access port.
After I finished the install decided to clean & drain the tank. Realized I could put a hose in thru the window and siphon it out. With hose in tank, slowly turn on hose attached to faucet-once air is out turn off water un screw hose from faucet (keeping it lower than tank) and tank should drain. WOW like in 15 minuets instead of normal hour or more using the tanks drain. Plus you can move hose around to suck any debri off the bottom.