Definitely a template.
Also, decide on a reference point or points and make careful measurements. To do this, pull a string from dead center aligned with coupler/tongue jack to exactly the middle between the rims or center of tires as reference.
Pull this line all the way from bow to stern. Staple in place while taught.
Now, measure each bow to stern point from this reference point created by the string line. Write in permanent marker on the floor or on paper which you tape or tack down. Once all necessary measurements are documented as above, take careful pictures of the written measurements beginning at bow, street side (port side) moving to curb side (starboard side) of frame.
Keeping this sequence then labeling the pictures in the grid format you will have the best chance of close reproduction and benefit of future diagnostics when you have a problem pop up after build completion.
Use same logic for wiring and plumbing and label. Pictures properly.
Do not forget to document the screw locations as you may be able to reuse them and they will help verify measurements too.
There are optional products to consider beyond plywood, too. More expensive but they may outperform and outlast the plywood. However, if you epoxy the new plywood you may be just as happy.... By the time you use the wood and epoxy you may be close to cost for the new synthetic. Your call.
One product called Seaboard is quite promising. There are other with similar performance. Bottom line is NO rot, a bit heavier, not quite as flexible as wood, but take a look and decide for yourself.
Also if you have good measurements you could layout the floor in your workshop while you are having frame repaired and painted. .
Peace and Blessings..