Begin with two hits on the lowest floor tom and two hits with the bass: RLFF and repeat 4 or 8 times (depending on how comfortable you are), then to RLFFFF (of course, you are alternating your feet with the double-bass), then 6 hits with the bass drums; RLFFFFFF... then move to the next drum playing two hits and two on the floor plus the bass: RLRLFF, then RLRLFFFF, then RLRLFFFFFF, etc. Onto the next higher tom; two hits on that, two on the low tom and two with the floor tom plus the bass: RLRLRLFF; then RLRLRLFFFF; then RLRLRLFFFFFF, etc... depending on how many drums you have, you can keep this up for a while. Notice how the time signatures shift, if you are able to keep it going on a continuous fashion. The idea here, is to keep the cycle of notes going constantly, without stopping. Start at a reasonable tempo and make sure your hand/foot technique and your posture are good.
After you've achieved a smooth and flawless tom/bass permutation all the way UP your drums, then you have to do the opposite starting with the left hand and left foot: LRFF/LRFFFF/LRFFFFFF; LRLRFF/LRLRFFFF/LRLRFFFFFF...etc. Once you can do these things with both hands and feet, I'm pretty sure you won't struggle with tom/bass combo fills again.
This is a Terry Bozzio etude from his early 90's video.