How do I open the cap for the water tank. Have unlocked it, & turned, but nothing happening. This is for an Elldis caravan

Check the expansion tank for proper level of glycol heat transfer fluid. When hot it must be between the low and high level markes. Your expansion tank may be hidden behind a panel depending on manufacturer.

If level is low, you may have an air bubble in the system. Add antifreeze and find circulator pump. Turn pump up to high and set thermostat to highest temperature to force pump to run. Open bleeders found in one or more places in heating system to release air pockets. Leave pump on for 1/2 hour or so to gradually remove all air from system and release through expansion tank. Refill tank as necessary keeping level between high and low marks.

Turn pump speed back to normal for your comfort - mine gurgles in expansion tank if set above 2 on the adjustment knob found on face of pump motor.

If you fill the tank completely when the water vents you'll see were it comes out . Usually a hose from top of tank going down the side of tank then through floor and will have a screen on end to keep the bugs out.

problem is there is an RV for your system. you will need to do the following.
Drain all holding tanks, fresh water and sewage

1. Attach garden hose to fresh water fill and fill tank.
2. Turn on pump switch and open cold water side of all faucet/ shower fixtures. Leave open until water runs clear. Repeat for hot water side.
3. Flush toilet until clear water runs into bowl.
4. Dump tanks again.
5. Sanitize water system. use a water sanitizer
6. If a water filter is installed, drain lines, remove filter assembly, clean and reinstall with new filter.
7. When ready to use the water heater, turn by-pass valve to open position to allow water to enter and fill the hot water heater tank.

The water tank drain should have a shut off valve to prevent water leak. I wouldn't cap it, you should want to drain water tank on occasion.

Swift Caravans continued the production of Abbey Caravans until 2008, when the brand was dropped along with Ace in order to simplify the Swift offering.
Perhaps attempting contacting Swift Caravans with your inquiry:

Perhaps Contact Us

what do you mean by unclog? If you mean when you open the dump valve and nothing comes out but you know the tank is full then I suggest this hydroflush. It works pretty well for cleaning out the tank even if it isn't clogged as long as you don't have too many bends or fittings between the drain cap and the valve.

https://amzn.com/B0002UHVAA

U SHOULD HAVE A BLACK TANK AND A GRAY. TOWARDS THE BACK WERE U HOOK UP THE HOSE FOR DUMPING U WILL SEE TWO LEVERS ONE IS FOR BLACK ONE IS FOR GRAY. AND IF UR TALKING ABOUT FRESH HOLDING TANK LOOK UNDER NEATH AND U SHOULD SEE A PLASTIC NOSEL WITH A TURN VALVE LIKE WHAT BOX WINE HAS TO PURE A GLASS

Yes. Open the outside panel, usually located close to or beside the city water inlet. Should be a diagram showing "normal", "tank fill" and "winterize." Use "fill" to fill the tank. Water will run out the underneath drain when full. I recommend a water pressure limiter just in case the city water pressure is excessive. You can rupture a line or the tank if too high. Rare, but it does happen.

Flip the valves to "normal" to use the city water. If boondocking, the pump will pull from the fresh water tank directly. No need ot change anything.

Fresh water holding tank will have a single 1/2" line hanging down under the trailer. It will have either a cap or a valve. To drain open the valve or remove the cap. For gray water and black water you must be connected to an approved dump site with an approved RV sewer line. Open the 3" valve first and allow the black tank to drain then open the 1-1/2" valve to drain the gray water. This will flush the sewer line as well.