Buy good new
drill bits, and run them at a lower speed. Too many folks think that going faster makes the drill cut faster...NOT!
HSS (high speed steel), cobalt, or TiN coated HSS do well enough.
BTW, HSS refers to the drill bit metal itself, NOT the speed at which you drill the holes!
Slower is better! BTW, rusty metal dulls drill bits faster than clean metal will. If you can, wire brush or grind/sand off some of the surface rust before drilling.
If the drill bit is dull either have it sharpened or just buy another bunch of them...they ARE essentially disposable, after all.
Trying to drill with a dull bit will just HARDEN the metal, and make it difficult if not impossible to drill out.
Remember: slower speed, high pressure, watch you don't break the bit!
Large round holes:
I use a hole saw (I have several sizes I use in my handyman biz), and make sure it's a bi-metal one made to cut metal. Essentially a very large bit that cuts out a circle.
Use VERY slow drilling speed (I use a 1/2 inch electric drill), LOTS of pressure, and a bit of cutting oil does the job. Deburr the cut (sharp!) edges afterwards, clean off the oil, prime and paint to reduce rusting problems.
Drilling holes and making them larger:
Start small! Then use gradually larger bits, and take your time.
This process is called "step drilling".
If you're trying to use old floor holes for attaching the ply, maybe get under the bus and drill UP!?!? through the old holes. Use GOOD eye protection, and have someone nearby to take you to the emergency room when a chip gets in your eye.
Go to boneyards!!!! I've purchased small RV fridges for $20, larger for $50-100. RV stoves for $20. Water heaters for $20. Rooftop AC units for $50.
Caveat: buy with a promise to refund your $$ if it doesn't work, OR fire it up on propane/electric/whatever at the boneyard before hauling it away.
I use a 20 pound bottle with a gas grill regulator to try out propane appliances; mounted on a handtruck it's easy to transport everything around in the yard. Works like a charm.
Bought a 19 foot RV awning with a good cover for $100, new they're $500 and UP.
Salvage a bunch of stuff from the same RV and maybe the guy will give you a better deal. Remember the fresh water tank(s), water pump(s), charging system(s), etc. Lots of stuff gets crushed instead of reused......
Look around. Find ALL of the boneyards in your area. Go back to 'yards, they get 'new' stuff all the time.
And negotiate prices: just don't be a dick about it. Be reasonable!
Call local body shops, tow companies and insurance places, maybe they know of an RV that got wrecked and they will sell for short $$ to recoup some dough.
use "SO"-type electrical cord, that's heavy extension cord to y'all!
NEVER use solid wire; use ONLY stranded wire! It remains flexible when our buses vibrate and shake. Solid wire fatigues and breaks.
A licensed electrician buddy told me to use "SO" and tin/solder all wire ends for better trouble-free connections. Use Rosin-core solder only, as acid-core solder will just eat the wire connections over time.
ALWAYS attach the green or bare ground wire to the bus chassis as a ground for electrical safety when using 120VAC/240VAC systems.
Look over the Index here for conversion techniques for a "sticky" on bus wiring, plumbing etc.
Spend some time looking around, don't be lazy, OK?
Lots of info already here SOMEWHERE.