Hi OP, what age of child is it you're looking for a child seat for? Is it a newborn? With some cars when they have underfloor storage in the back, it's an option to open the box up and extend the leg of the seat right to the floor, so there are some which are longer. However I don't know what the typical distance of this would be. It might be you can drive the motorhome to somewhere that you can try different seats or bases to see if any would be longer, or perhaps take a tape measure into the store. The max length of support leg from the seat edge to the floor is 525mm according to the child safety seat standard, so you are probably cutting it pretty fine to find something which will reach. 85mm is allowed above the edge of the seat, so you might find that Besafe seats with the adjustable tilt fit adequately, e.g. izi Turn / izi Twist. Those are suitable from birth if you have the B version. They are not cheap seats though.
It might also be possible to raise up the floor or attach something solid such as wooden boards to provide a raised but solid base for the support leg to rest on. You would need to contact the manufacturer of your current car seat(s) and see whether they would be agreeable to this as a solution. I wouldn't do it without a technical sign off from the manufacturer, though.
As others have said, fitting with seatbelt is also a good option, and there are plenty of seats which do this well. Seatbelt fitting is no less safe as long as it is done properly. I would recommend looking at Joie seats.
There are a few seats which are suitable from birth and use a top tether instead of a support leg, but no infant carriers at least for the UK/EU regulation, and most of these seats have downsides such as lower rear facing limits or not being well rated for safety. If your child is older, it might be easier to find a suitable seat.
There are some other things to be aware of when fitting a child seat in a motorhome. The seats have to be designated as passenger seats and meet all requirements, seatbelt must meet ECE R16, isofix needs to be a proper fitting compliant with ISO 13216. There are "universal" isofix kits sold online which are not compliant and they have been found to break in crash testing, which ejects the seat. If the isofix or seats are not original to the motorhome, you might want to check this. The easiest tell is that proper isofix has smooth, round metal bars bent into two square U shapes, which you should be able to feel. The aftermarket kits tend to have squared edges as though they are punched out of a sheet of metal, which is not as strong. I've just mocked up a really quick graphic to show the difference between a compliant part both installed and uninstalled, and the fake kits. I don't know if there are other fake kits around which look more like the compliant ones. But if you have any doubt at all, don't use the isofix.